He tells of how he was driven to the brink of suicide in Stephen Fry: The Secret Life Of The Manic Depressive, on BBC2, at 9pm.

He was diagnosed with bipolar disorder at the age of 37 shortly after walking out of a starring role in a West End play.

For one of television's best loved performers, his spectacular collapse became headline news, but for many it is a condition which goes undiagnosed for years.

For one Peterborough mother, it was watching her son withdraw from life as he suffered with the condition, but not knowing why which was hardest.

She has asked not to be identified to protect her family, but hopes the documentary will help lift the taboos of mental illness.

Her son was diagnosed at the age of 18 following months of worry.

It took more than two years to overcome the condition, even spending a period of time in hospital.

"GPs are marvellous, but they don't seem that well trained to spot mental illness," she said. "I think there could be more places for people to go – it's a huge jump from a GP surgery to a psychiatric ward.

"You feel really helpless when they are in hospital. I was visiting all the time. But probably just visiting can be daunting for some people."

And she added: "Whatever people's behaviour, it's important to get help and not to give up on them – most of the time they don't remember what they were like when they were unwell, so for the person caring it can be a lot harder."

The main symptom of bipolar disorder is mood swings, from extremely happy, called mania, to extremely sad, known as depression.

During a manic episode, people often cannot tell anything is wrong and might feel as if other people are being critical, negative or unhelpful.

The more common symptoms can be incoherence, rapid or disjointed thinking, being talkative, severely impaired judgement, ever-changing plans and ideas, constant elation or euphoria, waking early, and highly energised, and promiscuous, or increased, sexual behaviour.

Depression will often follow, compounded by the trauma of a manic episode.

Everyone gets depressed or feels down occasionally, but with bipolar it is far more than the feeling of being fed up and miserable. People feel useless, desperate, guilty, hopeless, isolated, empty, demotivated, tired, and unable to think properly.

They may also find it difficult, if not impossible, to do the most ordinary things of everyday life.

For the Peterborough teenager however, his main symptoms were weight loss and depression.

His mum said: "He just withdrew from life. He had spurts of depression on and off. He had a few traumatic experiences in his life and became very withdrawn and quiet and was off his food – then other times he was really outgoing.

"We were backwards and forwards to the doctors for ages – they picked up on his depression, but not the condition."

And she added: "It's a very difficult thing for anyone to diagnose. A lot of the symptoms people might just have put down to him being a moody teenager.

"It was months before he was diagnosed. The worst time was not knowing what was wrong."

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