A few weeks ago, Megan Stephens got on a bus in a bustling city centre in the north of England. A man sat across the aisle from her. He was wearing sunglasses and had a moustache. For a horrible moment, she thought she recognised him.

“I just froze and missed my stop,” Megan says. “I was using my phone as a mirror to see if it was him. I was really paranoid.”

The man on the bus had exactly the same features as someone from her past. As a result of what that person did to Megan, I am not allowed to use her real name or describe where she lives. I can tell you that she is 25. Other than that, she has asked me not to mention any details which might undermine her anonymity.

Every one of Megan’s days is shaped by the fear that she will be discovered and that her true identity will be revealed. This is because 11 years ago, at the age of 14, Megan was trafficked into the sex industry.

According to the United Nations, she was one of an estimated 2.4 million people around the globe who are victims of human trafficking at any one time, 80% of whom are being exploited as sexual slaves. One woman can earn a trafficker between £500 and £1,000 a week and can be forced to have sex with multiple partners in a single day. Megan, however, claims she used to net her abusers a similar figure each day.

The man Megan saw fleetingly on the bus reminded her of one of her traffickers: “I felt more scared than I thought I would be,” she says. “I was on a bus full of people in the middle of the city and I was terrified. Absolutely terrified.” It wasn’t him.

Megan’s story is a horrifying one. It is a story of how a vulnerable teenage girl on holiday in Greece with her mother was trafficked into the sex industry and spent six years as a prostitute – in brothels, on the streets, in dingy hotel rooms – before finally making her escape from a life of relentless physical and sexual abuse. It is horrifying not only because of the sadistic violence she endured, but also because of how easily she seemed to slip into this spiral of depravity and how difficult she found it to get out.

“Your whole identity is robbed,” Megan says. “Unless you’ve been in that position, you can’t understand.”

We meet in a beige hotel, chosen for convenience and its lack of defining features. We are here to talk about Megan’s memoir, Bought & Sold, which has been produced with the help of a ghostwriter.

The ghostwriter, a kindly woman called Jane, sits with us, to provide reassurance.

Outside it is dark and raining. Megan drinks a cup of instant coffee as she talks. When she speaks, her words seem curiously disconnected from the overall neutrality of her demeanour. It feels as though I am looking at her through a pane of glass – her eyes are veiled, the lines of her face set deliberately not to show too much emotion. There is a dissonance between what she is saying and the way she is saying it, almost as though the only way she can get the sentences out is to be as calm and matter-of-fact as possible.

Megan says writing the book was “therapeutic” and helped her rediscover her voice. In a different life, she would have liked to have studied English literature at university.

“When I came out of it [the sex trade] I couldn’t speak to anyone,” Megan says. “I had no confidence. I flinched when someone shouted. I’ve been diagnosed with post-traumatic stress disorder by a psychologist who talks about how I disassociate from my past. For me, it’s like it happened to someone else.”

Megan had a troubled upbringing. Her parents divorced when she was four and both her father and mother had problems with alcohol. Her childhood was chaotic and punctuated by fights: “I was never taught boundaries or rules or life skills.” At the age of 14, she went on holiday with her mother to Greece. She remembers, at the time, being “desperate to be loved”.

So when, on the first night away in a local bar in a seaside town, Megan caught the eye of Jak, a handsome Albanian man, and he started paying her attention, she responded. Within days she felt herself to be in love. Within weeks Megan had persuaded her mother not to return to England and had set up house with her new boyfriend.

Athens by night: Megan’s trafficker moved her to the capital where she was forced to have sex with many men every day. Photograph: Alamy

Why did her mother allow it? Megan shrugs. “She was not a well person back then.”

In the book Megan recounts how her mother had also struck up a relationship with a local bar owner. Greece seemed to offer them both the opportunity to start again. Her mother moved in with the bar owner; Megan moved in with Jak.

“When we left England we left our lives, really,” Megan says. “There was nothing left behind for us.”

Jak, dark-haired and dark-eyed, was attentive and kind at first, despite the language barrier which meant that neither of them could communicate beyond a few words. By her own admission, Megan was deeply naive.

“He treated me so well,” she says now. “I just believed him. I loved him and he loved me pretty much instantly. He was charming, really.”

But as time went on, this “charm” turned into control. Jak’s moods could shift without warning. He started talking about how his mother was ill with cancer and how the family needed more money for treatment. He told Megan he dreamed of having children with her, of living in a nice, big house in the future. In order to make that happen, he explained, they would have to move to Athens, where his cousins could get them café work. Megan agreed, even though it meant leaving her mother behind.

But the café work turned out to be something else entirely, and once they got to Athens, Megan found herself at the mercy of a network of pimps and traffickers. At first she wasn’t sure what was happening. It was only when Jak gave her a cardboard box and deposited her outside an office building telling her to deliver it to a man on the top floor that she began to suspect something was awry: “I remember shaking and stumbling up the stairs, because something felt odd.”

A man opened the door to her, took her into a small, windowless room with a single bed. At the foot of the bed was a video camera mounted on a tripod.

“And that was it,” Megan says now. “He… just… raped me, really. He was filming it and I was paralysed, because I was really shocked.”

Afterwards, with blood on the bedsheets, the man gave her a wad of €50 notes. As Megan was leaving, she saw the cardboard box she had been asked to deliver contained several packets of condoms. It was the first time she had ever had sex.

What, I wonder, would the Megan sitting in front of me today say to that scared teenage version of herself if she had the chance?

“I don’t know… ‘Get out, you stupid girl’?” she says, phrasing it as a question. “I still blame myself. I’m struggling with it. I’ve got quite a lot of anger at myself.”

But Megan didn’t get out. She began having sex with strangers for money – up to eight “clients” a day. She was in love with Jak, she says, and would “do anything for him”. He made her think that escort work was the only way to raise enough money for them to be together. He would shower her with affection one minute and, the next, humiliate her in public. If she said she wanted to stop, he would threaten to kill her mother. Gradually her confidence was eroded to the point of no return. She was utterly reliant on Jak and his network of underworld associates for everything: clothes, food, transport.

For a while she was a streetwalker in Italy (“That was horrible… I was scared of the other women as well as the clients. They were very, very tough characters”) and then she was forced to work in a series of brothels where men would pay €20 for a grubby, two-minute encounter. “It was just the way they operated,” Megan says. “They [the men] were queuing up outside. There were 10 to 15 rooms in the same place and it’s just… literally, you don’t stop… If I did 40 to 50 people, that would be nothing. It wasn’t enough.”

On one particular night, she says she had sex with 110 men before being violently sick. The owner of that brothel closed up early when he saw how ill she was. “I thought that was decent of him,” Megan writes, “which shows just how distorted my sense of normality had become.”

In the book Megan’s narrative seems to exist outside normal chronology. She was in a mental fog for much of it. She was ill – underweight and exhausted. She contracted syphilis and salmonella six times. And if she misbehaved, there was violent retribution – on one occasion, she was punched in the face by Jak and dragged across the floor by the roof of her mouth. “Things like that happened all the time,” she says blankly. “I can taste the blood even now.”

At some point, Jak left and handed her over to another pimp called Christoph, who moved her around wherever the work might be – from hotel to brothel to private apartment. All the time her captors told Megan to send postcards to her mother (who was still living with the bar owner in Greece) telling her she was working in a café and happy with her new life in Athens. She agreed because she felt helpless and didn’t want to put her mother in danger. She was also ashamed.

“These traffickers are really, really clever,” Megan says. “I want people to understand it’s not as easy as getting up and leaving. I should have got up and gone, but I didn’t because of the mental power they had over me. It is really powerful. It’s actually like they’ve taken over what identity you have and turned you into their property, a thing to be controlled. Robotic is the right word.”

This seems incredible, especially when Megan writes in the book that she helped a Polish girl escape by asking a rich client to book her a plane ticket back home. She says it simply never occurred to her to do the same for herself. Her own sense of worth had been diminished to such an extent that she no longer knew her own mind. And she was still only a teenager. She had been given no chance to grow into an adult capable of making her own decisions.

‘I don’t value sex at all. I think it’s horrible’: Megan on the legacy of being trafficked. Photograph: Alamy

Megan was picked up a few times by the police, but was too frightened to tell them the truth in case they were in league with her abusers. She didn’t trust authority. “I was so, so paranoid,” she says. “At that point, I was scared of being killed.”

Eventually she suffered a psychotic episode and was sectioned in a Greek hospital for three months. Cocooned from the outside world, she began to feel safe enough to confide in some of the staff about what had happened to her. They contacted Megan’s mother, who, in spite of living just hours away, said she had no idea about the kind of life her daughter had been living. The two were reunited shortly afterwards. What was that like?

“Really I was just zombified because I was on so much medication. I was emotional. All I wanted to do was go and drink, and I definitely didn’t want to talk about it.”

Megan and her mother returned to the UK. A doctor put her on Prozac. For a long time she struggled with everyday existence. She was scared of crowds. She jumped at loud noises. She couldn’t find the words to explain what she had been through. She turned to alcohol as a crutch. She spent too much money and had a series of bad relationships.

“Inside I still feel like a kid, a 10-year-old,” she says. “I struggle with sex. I do not know what ‘making love’ is. Just… it… that way… it…” she fumbles for the right word, “it just makes me feel so odd, so different and not normal. There are relationships I have been in where I’ve had to be drunk to let anyone see me naked or let them do what they want to me. I struggle to say no to sex because I thought that was all men wanted. I actually hate that. I don’t value it [sex] at all. I think it’s horrible.”

Eventually she found the confidence to get a job as a shop assistant, and she confided some of her story to a colleague, who notified an anti-trafficking charity. The charity got in touch with Megan. Within days she was in a safe house in London.

“After all the turmoil and chaos I had been used to,” she says, “it was like living in a calm, well-organised family home.”

Today Megan is cautiously rebuilding her life. She has ambitions to set up a charity of her own to help trafficking victims like herself. She is in therapy and has been alcohol-free for seven months. She has a group of trusted friends, made through her local church, and she is rebuilding her relationship with her mother.

Does Megan blame anyone for what she has been through?

There is a long pause. “I don’t want to sit here and say: ‘I blame my mum,’” she starts, uneasily. “I believe my upbringing could have been better and I should have been protected more as a child, but I understand why that wasn’t the case.”

It is interesting that she doesn’t immediately point the finger at her abusers and a sign, perhaps, of the complicated intermeshing of love and fear she experienced at the hands of the men who exploited her. She confesses that, shortly after returning to the UK, she called her former pimp, Christoph, “because I just… I actually felt in love with him, I did. I look back and it’s horrible. I felt trained into it.”

It is only recently that she has finally felt free from that mental imprisonment. And yet the young woman in front of me is still clearly damaged, existing at one defensive remove from her own past. She isn’t yet sure how to be, or what kind of person she is when she’s not living in a state of constant terror.

I ask Megan to try to describe herself in three words. She finds this difficult.

“Strong,” she starts, hesitantly. “I feel strong.” A pause. “Determined. Could that be one?” she asks. I nod. “Yeah, and… hopeful,” she adds in a small voice. “That’s me.”

Bought & Sold by Megan Stephens is published by Harper Element at £7.99 on 29 January. To order a copy for £6.39, go to bookshop.theguardian.com