Kelly is 23 years old. She is a bright and thoughtful young woman with an attractive smile that flits across her face occasionally as we spend several hours discussing her background, her family and her short life.

With her long, tousled, chestnut hair and short, checked red dress, she looks like many of the 50,000 cheerful students who throng the bustling Belgian city of Leuven, home to the country's biggest university.

Yet there is a big difference. For this friendly woman sitting on the sofa beside me – someone younger than both of my own children – is telling me an anguished tale of deep personal torment.

'When I look in the mirror, I see a monster,' she says at one point, although dark rings around her eyes are the only outward sign of her struggles. 'I really don't like what I see.'

Kelly aged 11, right, and left, now, aged 23. She says she has never had happy days in her life

Kelly (pictured with boyfriend Bregt) is plagued by mental health problems that she describes as being like a knife plunged in her chest – and we are meeting to discuss her determination to die

Such words do not seem to match the person speaking them. But Kelly is plagued by mental health problems that she describes as being like a knife plunged in her chest – and we are meeting to discuss her determination to die.

'It won't hurt so I don't see the problem,' she says.

'It is like going to sleep, and for the first time in my life it will be quick. I won't be lying in bed waiting for it to come.'

In almost every place on the planet, this would be a huge problem. But Belgium is one of three countries that permit such euthanasia, making no distinction between 'unbearable' physical and mental pain. The other two are Holland and Luxembourg.

Yet even in Belgium – which, 17 years ago, became the world's second country to sanction euthanasia and the first to legalise it for children – psychiatric cases remain controversial, especially when they involve someone as young as Kelly. Mental health patients account for about three per cent of the 17,000 people killed since the law was changed in this country of 11 million citizens. There were 2,357 deaths last year – ten times as many as in the first year euthanasia was legal in Belgium – and most involved elderly people.

Many psychiatrists and most Belgians oppose extension of euthanasia to mental health cases. Some experts argue diagnosis is subjective, unlike severe physical illness, and insist the lives of distressed younger patients can improve with time, therapy and medication.

Yet Kelly, whose birthday is this week, plans to die as soon as legally possible so has no time for such arguments. 'It feels like discrimination,' she said. 'It makes me angry. It's just not fair. They don't understand the pain.

'People look at me and see someone so young but I feel bad inside all the time. It is not the age that is important but the suffering of the person.'

This woman – so open and yet obviously troubled – highlights the complexity of this issue as Britain faces growing pressure from some patients and politicians to permit doctor-assisted death.

Opponents argue legalisation is a slippery slope.

Kelly lives with her family near Leuven – yet incredibly had not told her parents, identical twin sister or younger sibling of her plan. She was due to break the news to them four days after our discussion, something she admitted she was dreading.

'It is not going to be easy to tell them. I think my sisters will understand more than my parents. But it will obviously be very difficult, especially for my twin sister.'

The reason for her failure to tell her family soon becomes clear. She is fond of her father, a salesman, but said her relationship with her mother, a former cleaning lady, was very difficult, leaving her feeling unloved and insecure at home.

'As a child I did not understand what was happening,' she says.

Her stresses were fuelled by crippling shyness.

Kelly lives with her family near Leuven (pictured) – yet incredibly had not told her parents, identical twin sister or younger sibling of her plan

This was intensified by living in the shadow of a more extrovert twin sister – which allowed Kelly to withdraw into a shell – and frequently changing schools as the family moved between rented homes. The twins were often identically dressed, even as teenagers, although their parents made some efforts to encourage their individuality.

'My sister did everything, such as making friends for us, so I never really had to do these things for myself. It felt like we belonged together, although I now see we were very different.

'But we were always together. Then when we were separated at school, I found it difficult.'

Kelly focused her efforts on working hard and trying to please her teachers. But she says she always felt lonely and unhappy – and even when she did bond with new friends, it seemed as if they were drawn to her vivacious sister. 'I tried telling her once how I felt, that it felt like she stole away my friends. I don't think she understood.'

I asked if she had ever been to a party? 'Yes, I went once and it was awful,' she replied. She tried asking a few friends to her house for a sleepover around the same time, when she was 16. 'But only one came, so I felt rebuffed.'

These are, of course, standard social anxieties for many teenagers. And for all her doubts and insecurities, I found Kelly to be very affable and straightforward – despite evident anger beneath the surface – once she began to relax during our discussion. Yet trust is clearly an issue after her disruptive upbringing. So despite pouring out her upsetting life story in detail, she was scared to tell me she had a diagnosis of borderline personality disorder for fear that it would change my opinion of her.

I told her it was hard to understand such psychiatric trauma for those that do not suffer.

'It is like physical pain,' Kelly replied, holding both tightly clenched fists to her heart. 'It feels like I am breaking apart.'

Things began to spiral out of control at the age of 18 after the death of a beloved grandparent combined with a feeling of betrayal by a female psychologist she had grown to trust. 'I could not go to classes. I would sit in the toilet all day crying.'

To win the right to die, Kelly (pictured) – who has been unable to work since leaving school beyond a bit of babysitting – needs backing from two psychiatrists and one other doctor

Kelly attempted suicide, was hospitalised, suffered eating disorders and started to self-harm.

'It was easier to feel the pain from self-harm then the pain in my head. At least the pain from self-harm goes away, unlike the pain inside that is always there.'

She decided to apply for euthanasia after learning it was lawful and possible from a psychologist in the hospital.

'I felt 'yes',' she said, pumping her fist as she recalled the moment. 'I immediately went and looked up all the information I could find.'

To win the right to die, Kelly – who has been unable to work since leaving school beyond a bit of babysitting – needs backing from two psychiatrists and one other doctor. They must agree her mental pain is unbearable and untreatable.

She applied seven months ago and is being assessed by Joris Vandenberghe, a local professor who has helped draft stricter rules for psychiatrists amid concerns that some patients died despite treatment options being available.

In the end, only her father attended the meeting on Friday with her doctor to break the news.

'My father was very shocked, he cried and I grabbed him. I was almost crying, too. It was very moving but also painful to see him like that,' she said.

Vandenberghe declined to discuss Kelly's case.

But he recently wrote an academic article admitting that Belgium's policy was 'highly controversial and raises difficult ethical and clinical issues'.

He argued its laws had failed to ensure 'sufficient checks and balances to promote reluctance to act on a patient's death-wish', concluding that more investment in mental healthcare could prevent some, but not all, of the demand for euthanasia from distressed patients.

Last year it emerged that three Flemish doctors, including a high-profile psychiatrist accused of being behind almost half the cases of euthanasia for mental health disorders, were being investigated on suspicion of 'poisoning' a woman who had autism.

Kelly attends a fortnightly group session with five others undergoing this vetting process where she met her boyfriend Bregt, 44, a father-of-two who grew up in an abusive and dysfunctional household, leaving him suffering post-traumatic stress disorder.

'I have always felt I don't belong, that I don't want to be here. I try to conform as wanted by society but it does not work,' he said. 'There is this constant feeling of being an alien, a freak.'

The pair have discussed whether they should die together if they gain consent, but are conflicted. Bregt, a former social worker, fears he will not have the strength to wait. Kelly admits that while she would like to 'do it' together it might upset her family.

'I don't want my family there. I have felt so lonely for so long, why should I have people with me when I die? I'm just going to sleep – woo-hoh!' she added with a smile.

The group sessions are run by Ann Callebert, a psychologist who has had her own mental health struggles, including feeling suicidal. She said: 'My aim is to show life can be bearable, to find ways to enjoy life, since they have the feeling it is too painful.'

She admits it is difficult when patients go on to be killed by euthanasia – and that Kelly, the youngest to join the group, has got under her skin.

'I can feel her suffering. I would not want to be in her head.'

So does she want Kelly to win consent to die?

'That is difficult. On one side there is the constant hope she can make something from life, something to give meaning. But then I can see her daily fight and I would love to find a way to take away the pain.'

As Callebert says, Kelly is 'a tough girl'.

It was disturbing to talk in such an open manner about the possible death of someone so friendly and physically in their prime. But equally, her intense mental difficulties were clear to see.

Kelly has contemplated what she will do if denied euthanasia. 'First I will try to find another doctor, but if that does not happen then I will give up. That means suicide.'

Last year I interviewed a Belgian woman with autism left seriously traumatised after being locked up in psychiatric units during childhood. She had won the right to die – but since being granted control over her death had stopped thinking about suicide.

So could this happen to you, I asked Kelly.

Could you possibly find any happiness in the future?

She shakes her head in response, before saying quietly: 'Happy days in my life? Never, seriously never. I want this all to end.'

As she left with Bregt, I gave her a big hug and hoped that she was wrong.