It's the end of the day at the end of a week that Tulisa Contostavlos hopes she never has to go through again. She sits down, and exhales like a slow puncture. "I've had to stay silent for a year," she says. "I've had to sit back and look like the bad guy, while my whole life was ripped apart."

In June 2013, Contostavlos was "exposed" by the Sun on Sunday's Mazher Mahmood, aka the Fake Sheikh, in a front-page splash alleging she had fixed a cocaine deal for him. Even by Mahmood's standards, it was an elaborate sting, involving first-class plane tickets between London and Las Vegas, luxury hotel suites, the prospect of a £3.5m film contract opposite Leonardo DiCaprio in a big Bollywood production. You couldn't make it up – only Mahmood did, and Contostavlos bought it.

Since then she has lost more than a stone, changed her physical appearance, ditched (or been ditched by) nearly all her colleagues, been abandoned by friends (and found a few brilliant new ones), had her endorsements frozen, lost any number of contracts and contemplated suicide.

At the outset of the trial last month, her friend Michael Coombs pleaded guilty to providing Mahmood with £820 worth of cocaine. The Sun on Sunday printed texts in which she offered to "sort" drugs, and filmed Coombs arriving at the Dorchester hotel in London to deliver them at a later date. In court, Contostavlos proclaimed her innocence but could see only one outcome. "My heart told me: there's no way this can happen. I'm very religious and I kept saying, God won't forsake me." And her head? "My head was telling me, if they're allowed to take it this far, to tell so many lies, God knows where it can go." She thought she'd be convicted? "100%. I was preparing for prison."

But on 17 July, four days into the hearing, it emerged that Mahmood may have lied in a pre-trial statement. His driver, Alan Smith, had told police Contostavlos had talked about how much she disapproved of drugs when he drove her home one night, because she'd seen the damage it had done to a relative. Smith had been expected to give evidence supportive of Contostavlos, but later changed his mind. Did he discuss his statement with Mahmood? In a pre-trial hearing, the journalist denied this; but under cross-examination in court, he admitted he had received the driver's statement and that they had talked about it.

The case was thrown out: Judge Alistair McCreath told the jury there were "strong grounds to believe" Mahmood had lied on oath, and "the underlying purpose… was to conceal the fact that he had been manipulating the evidence". Proceedings against Coombs were stayed; Mahmood was suspended by the Sun on Sunday pending an internal investigation. He may now face perjury charges.

I meet Contostavlos four days later in east London. It is the first interview she has given to a newspaper in more than a year, and she has plenty to get off her chest. Only this morning she was in court for a second, unrelated case: she was convicted of assaulting celebrity blogger Savvas Morgan and fined £200. She is livid and insists she committed no crime – she will be appealing.

Contostavlos is perfectly friendly but has lost the easy smile and chirpy chutzpah of her X Factor judging days. She offers to share her pizza, shakes hands, and looks as if she wants to be anywhere but here – never more so than when having her photograph taken. Outfit after outfit is switched, the hair is brushed up, down, and still she is not happy. Occasionally she shakes and seems close to tears. She looks as if she needs a hug.

After nearly six hours, we sit down to talk. She is more relaxed now, finally pleased with the pictures. In a statement she made after the trial collapsed, Contostavlos said Mahmood's sting had come at a time when she was feeling especially vulnerable. What did she mean? Well, it was only a few months after an ex-boyfriend, Justin Edwards, had posted a private sex tape online. She made her own video in response, naming Edwards and saying she had nothing to be ashamed of. But she did feel ashamed: "I was very depressed and I blamed it all on the industry. I thought, if I wasn't in this industry, I wouldn't be experiencing this situation, which was an utter violation of my human rights. Humiliation. Just walking into shops and facing people was a task, never mind sitting on the panel of a national TV show going out to millions. I wanted to get out. I wanted to go into acting, and piss off to America."

She went to Los Angeles for auditions, which is where she was targeted by Mahmood, posing as a Bollywood producer. First she was approached by somebody claiming to work for 21st Century Fox, who set up a false Twitter account and followed her online. He then introduced her to Mahmood. She was flown to Las Vegas to discuss a major film, starring DiCaprio and backed by financiers who had a stake in Danny Boyle's Slumdog Millionaire.

"He was my childhood crush," she says of DiCaprio. "I'd admitted it in interviews, so they knew." They had done their research: she believes Mahmood and his team chose Bollywood because they knew Gareth Varey, her close friend and PA, had worked in India as a dancer. "I think they'd studied the people he'd worked with, the movies he'd worked on, the places he'd been to – because that's what they based it all on. I flew to Vegas first class, picked up by limos, had two huge suites." Most of the time she was being secretly filmed by Mahmood. "When I first met Mahmood, I didn't have much idea what I was supposed to be doing. So you see on the tapes I start off very professional, just very myself and chilled out."

She made it her business to find out what the producers were after. "I didn't get as far as I have from sitting back and being dopey. I know what it takes to get a job, and I'm going to pitch myself. You look at the person who wants something and you listen to what they're saying. You pick up on it and begin to be everything they want."

They told her they wanted a British actress. "So I said, what kind of British? Posh British – and I start talking posh – or cockney? And he says, actually, we're looking for an urban, ghetto kind of British, someone with your accent. And my instant reaction was, 'Oh yeah, mate, I can give you all of that!'" She segues into street London, with accompanying hand gestures. "I'm pitching, fighting for a job. That's how I've got record deals, perfume deals, clothing deals. That's how I've gone into every meeting since the age of 11. There was a reason they called me 'the female boss' in N-Dubz – I did all the talking."

N-Dubz formed in 2000, an urban hip-hop group consisting of Contostavlos, her cousin Dappy and former boyfriend Fazer. It was Dappy's father who brought them together; the story goes that he offered the then 13-year-old Tulisa £20 to join the boys. It was only when he upped it to £50 that she agreed. She was talented, mouthy, but also insecure and struggling at home: her Greek-Cypriot father, briefly successful in the 1970s band Mungo Jerry, had left home when she was young, and she was an only child brought up by an Irish mother with schizoaffective disorder (she has often said she feels like the mother in their relationship). She self-harmed, smoked weed – and N-Dubz fans warmed to her honesty.

It was this drug-toking, street-savvy girl the fake film-makers wanted – even if that was a girl Contostavlos, now 26, had left behind some years ago. Most of the time she dealt with Nish, Mahmood's fixer. "We weren't even allowed to bring her to court, because it was said she wasn't a part of it. But for me she was a stronger character than Mahmood because she played the friend. She played on the motherly side: 'I read your book [her autobiography], everything you've been through with your mother is so sad, you deserve this. I'm the one who brought you to the attention of these guys because my daughter is a big fan of yours.' She played this very caring, almost guardian angel. She was the person I warmed to. I found Mahmood extremely slimy." Was he trying to be nice? "God, if that was him trying to be nice… I just thought he was a vile slimeball. And now I think he's 10 times worse."

Mahmood told Contostavlos that they didn't do official auditions, they did "social auditioning" – in effect, every encounter was an audition. Method acting, if you like. Nish, she says, cajoled her into becoming the bad girl. "She would make out she knew what I needed to do to get the job. So I would listen to her and think, oh yes, she's got my back." She says Nish told Gareth Varey, her PA, that the producers were choosing between Contostavlos and Keira Knightley. "I was told via Gareth that they really want the bad girl, and they're going to give it to Keira Knightley. So I had to prove the reason I'm better is because I am the girl in the role, and Keira Knightley could never be that."

Whenever she met Mahmood and his team, she became that person. "I went in trying to concoct stories from real-life situations. So I talked about a court case involving an ex but exaggerated it. I admitted I'd smoked weed at night every now and then. I'd not smoked weed in ages, but I was using that because it was the only thing I'd done."

On 10 May 2013, on a night out at the Metropolitan hotel in London, Nish suggested they go to the bathroom together; Contostavlos says she had been plied with alcohol for six hours. "It's just me and her, and in so many words she tells me, 'This is it, they want to give it to you, they want to give you £3.5m. You've just got to go in now and do whatever they want – be the bad girl, the ghetto girl.' The woman held my hand, looked me in the eyes through her fake green contact lenses, stared at me dead on, like I'm staring at you, and said, 'Tulisa, I want you to have this so badly, trust me.' I walked out of that toilet thinking, I've got this."

So she went into hyper mode, and her mild exaggerations became whopping lies. "All of a sudden I go from saying, hey, I smoke a bit of weed and there was this little trial with my ex, to hey, my ex is the biggest coke dealer in London and I used to shoot crack! I'd heard these terms from Dappy rapping rubbish back in the day. Come on, really!" She laughs. "I was drunk as a skunk. One of the things they disputed in court was how much I'd had to drink. Every time I finished one drink, they'd put another in front of me. And if I was like, no, I don't do shots, they'd be like, what d'you mean? Come on!' I felt under pressure to show I could drink them under the table."

She says they asked her to take drugs with them. She politely declined. She says Nish suggested she retire to the bedroom of one of the producers, which she interpreted as sex for payment, or at least payment in kind. Again, she declined.

As a number of Mahmood's victims have alleged, Contostavlos believes her drink was spiked. "Even though I was very drunk, for some reason I felt as if I was buzzing, on a high. Not pissed and slurry." She says one of the two friends with her passed out on the bathroom floor. "It's my belief all three of us were spiked." (Mahmood has strongly denied this in court.) "So it gets to the point where he keeps asking for drugs, and I've gone on a mad one where I've told him I can get him drugs." How many times had he asked? "Four or five. I was so trashed I'd made up this ridiculous thing about 'white sweets'. I was trying to sound like I knew about the gangster world. I'm thinking, if I start talking about white sweets, green sweets, pink sweets, he's going to think, hey this girl's down with the gangsters." She grins. "I'm smiling because at the time I thought it was funny. I thought, oh my God, I've got these guys completely fooled."

Contostavlos told them that half the guys she knew were drug dealers. OK, they said, put your money where your mouth is – which is when she panicked. She didn't know any dealers. She thought about who she could contact who'd sound convincing and landed on Coombs. There was a logic of sorts: she had acted opposite him in the TV series Dubplate Drama, playing a cocaine addict with Coombs as her dealer. Can she remember what she said to him? "Yeah: talk a load of shit to them, just make 'em think you're going to get it."

And he went ahead? She says she still doesn't know. That was the last time she spoke to him, though Mahmood's team filmed Coombs handing over a package at the Dorchester. "I wouldn't put anything past the Sun. It could have been talcum powder, and they could have done anything with it." After disbanding the case, Judge McCreath said that, because Contostavlos had been denied the opportunity to give her defence, it was only fair that he provide a summary of it: "She never intended drugs should be supplied to [Mahmood] by Mr Coombs or by anyone else. Anything which he [Coombs] did in that regard was out of a misplaced desire on his part to help her out of her dilemma, not because she asked him to do it; this was something she did not intend."

Did Contostavlos not think it unlikely that they were so keen to pair her up with DiCaprio, given her limited acting experience? "But the amount of money they spent, thousands of pounds, sending me to nightclubs in Vegas where I've got the best table, the biggest bottle of vodka, my own security guards. This is over about four months. I was like a lost puppy because I wanted it so badly to be true." She looks at Varey, sitting across the room from us. "He's the one who said, at times, something's not right. And I was, no, no, no, they're going to give me £3.5m, they've promised me Oscars, I'm going to be a movie star, I'm going to meet Leo. If somebody had told me what it really was at the time, I would have said, that's impossible, how is that even legal?"

Ten days after she contacted Coombs, Mahmood revealed his real identity. She says she can't even remember this, it was so traumatic, so her long-time publicist Simon Jones intervenes. "I'll tell you exactly what happened. I get a call, and he says, 'It's Mazher Mahmood, from the Sun' and I was like, 'Oh great! How can I help you?' He said, 'I just want you to know that your client is going to be on the front page tomorrow for selling me cocaine.' I said, 'Who?' And he went, 'Tulisa.' And I went, 'Tulisa Contostavlos?'" His voice rises in disbelief. "I actually went, 'Tulisa sold you some cocaine?' Mahmood said, 'I can't get into it any further, but we've already gone to the police, so I'd expect you to be hearing from them shortly.' I said, 'Woah-woah-woah, I just don't believe this, I'm going to have to call you back.' I couldn't get hold of Tulisa. I called Gareth and he said it's the movie people. It's the moment you just go sick in your stomach."

As he recounts this, Contostavlos does look as if she wants to be sick. Her head is down and she is hugging her knees, almost in tears. She felt humiliated at having been duped. "I wasn't even thinking I'd done anything wrong, or I was going to get arrested – I just thought they'd set me up to talk rubbish. That alone terrified me, that they'd print some of the shit I'd come out with."

Varey thought the splash "would be the straw that broke the camel's back". What did she think? "I hadn't got my head around it enough to want to go that far, but I was getting there. It was a big debate: 'Do I?'" She says it so casually, I'm not sure what she's talking about. Does she mean suicide? "Yeah. The fact it had gone as far as it had meant, to me, there's a higher power out to get me." Did she talk to her friends? "No, I wasn't going to terrify them. I've always suffered from depression. It's always something I've thought about, and it's something I attempted once when I was very young. People hear that and are like, 'Agh!', like it's a big deal, but it's not. It's a simple thought process: my life is ruined, it's over, do I stick around or do I end it?" She clicks her fingers. "It could last for a second or a minute, then I'll be all right again. The fact of the matter is, I'm still here."

Did she make a suicide attempt? "Maybe once. Something along those lines." She brushes it aside, and looks away. "Well, it wasn't a strong enough attempt. I didn't really know what I was doing. " She took pills and alcohol. "Luckily, they weren't enough to finish me off. That was my lowest. It was only three days after that I was back in a training session, ready to go to the studio."

What did she find most devastating, the loss of her reputation or the damage to her career? "My career," she says instantly. "What else have I had? This is all I've known. I've never been super close to family. I've never relied on anyone. I was left to my own resources and I just ran around and focused on my career. I have always been a lone soldier."

Over the past year, she's learned to be kinder to herself. "In most situations, you think you should have been wiser. But I don't because it was so extreme. The way those people sat and laughed with me and pretended to sympathise, and told me personal things, then listened to personal things about me – it's sick. I didn't think that, one, it was legal, and two, that there were people in such high positions capable of doing things that evil."

One thing she does blame herself for is getting Coombs involved. Before the case collapsed, he would almost certainly have received a custodial sentence. How would she have felt? "That would have been a very hard thing." She struggles for words. "I never wanted any of this. Mike is not a drug dealer." Throughout the legal proceedings, they were not allowed to talk, and when we meet they still haven't spoken about the case. "All we've done is cried on each other's shoulders at court."

She looks exhausted, I say. She nods. "The sleep hasn't been going too well." She still lives with Varey, who is gay and whom she has described as her "best friend, mother and brother". But they no longer sleep in the same bed, as they used to. "The way I've been sleeping, waking up screaming… I don't want a pity party, but it's not been fun." She has spent most of the past 13 months praying. "At least three times a day on a good day." And a bad one? "All day." She laughs. "I don't think anybody's made the sign of the cross as many times in one year."

She looks as if she's lost weight. "Yeah. Luckily I've put on a bit, but during the trial there was one point when I was 7 stone 11. [She is 5ft 5in.] It's not cute, is it? Not cute at all." What about her face – is she aware that people are suggesting she has had cosmetic procedures? Yes, of course, she's not daft. "Look, the papers have always said I've denied it, but I haven't given an interview in a year, so where have I had the chance to deny it? I just haven't spoken about it. Yes, I've had my lips done, and I'm happy with my lips. And I also had a tiny bit of filler in my cheeks. Pictures of me where my face was swelling, I had water retention – where you have filler your face draws up a load of water. So my face began to swell like a balloon. And this is when people thought I'd had surgery and said, what's she done? She's ruined her face."

I tell her I don't want to act the father, but she doesn't need all that stuff. "Thank you, I hear you," she says sweetly. "It was a choice I made. I'd lost a load of weight and I was looking at my face and felt…" She trails off. "It really bugs me when people say I've had a nose job. That's just makeup. Can you see the shading there? So losing a stone, filler in my cheeks, my lips done and the contouring – and people do think, oh gosh, you look so different. But I'm happy with it."

Anyway, she says, let's talk about the positives. Like what? Well, she says, she's going into the studio to record a single. Is she going to go on holiday, get away from it all? No, she's too much to do. "This is only the start. I've had my reputation ruined. Now I've got to fight to get it back." She stops again, clearly knackered.

The day the case was thrown out, she spent the evening at home with five close friends. Were they the same five friends she would have chosen a year earlier? "No. I've lost a good few friends and 90% of my associates. People in the industry who were so pally-pally – darling this, darling that – when the shit hits the fan they dissociate themselves. When they see you, they're like, 'Oh, hi' and looking round in case there's a journo seeing if you're stood next to Tulisa." Can she name names? "No. My mother always said, if you haven't got anything nice to say, then don't say anything at all. There were people I'd known for years who became closer than ever." Has Simon Cowell been in touch? "Well, me and Simon were never really that close. It was always like a work relationship, and I was never on the panel with him. I've never texted him in his hard times and he's never texted me. But I've never heard a bad word from him. People I was close to on the X Factor were Louis [Walsh] and Nicole [Scherzinger] and even Gary [Barlow]. Louis's always the first person to text. And [executive producer] Richard Holloway was very supportive."

Can she understand why Mahmood thought it a good story? "No." I play devil's advocate: you're a public figure, a role model for children, you end up fixing a cocaine deal and they want to expose your wrongdoing, even if that involves a degree of entrapment. Ah, but that's the point, she says. "If I was a drug dealer or a bad person, I can understand that. But there's a difference between entrapping a criminal and setting up an innocent person. "

Does she think they knew what they wanted from the start? "No, I think it was anything. As an industry person, I believe they assumed I took cocaine. Then, when they discovered I didn't, they did whatever it took." She is convinced there was an element of class snobbery – targeting the chav who flew too near the sun. "Other celebrities can be seen on camera with cocaine falling out of their pocket or hanging out of their nose. But certain people of a certain class can have one story and almost be seen as cool, whereas I supposedly gave someone a telephone number and am the biggest criminal of the year." She has lost a year's earnings. Will she sue the Sun? She looks at Jones. "Am I allowed to talk about this?" He shakes his head.

By now we have been in the studio the best part of eight hours. As Contostavlos talks, I find myself staring at the Female Boss tattoo inscribed on her elbow – the one she used to show off on the X Factor. Is this still an accurate epithet? "More than ever. I've surprised myself. I feel stronger, wiser – enlightened. I feel I've had 10 years' worth of life lessons in one year, and money can't buy that." She is still only 26, so how old does she feel in terms of lived experience? She goes quiet, as if she's totting up the years in her head. "Erm… erm… I'd say 60."

She tries to explain what she means about being stronger and describes sitting in court feeling her angriest. Mahmood was giving evidence from behind a screen and she kept shifting her chair to catch sight of him – to force him to look at her. She was boiling over, and didn't like it. "I reached a point when I thought, no, I'm not stooping to your level. I'm better than this. I looked at him and thought, he's not worthy of these emotions, he's draining me. I said, right, you're going to have to do it, and I was battling with myself, then I sat there and did it – I said a prayer for him."

If Tulisa Contostavlos were middle class, she wouldn't face such scorn