Tracey served part of her sentence at Holloway Prison
Too many people with mental illnesses are being sent to prison rather than receiving treatment, according to the Prison Reform Trust.
The trust says 90% of inmates have at least one diagnosed mental health disorder, and one in ten has a serious mental health issue.
More resources should be diverted into treatment rather than custody, it argues in a new report.
When 40-year-old Londoner Tracey (not her real name) went to prison, she did not think she had a mental health problem.
When she came out after serving four years of her eight year sentence, she knew she was not well.
"If you walk into prison without a mental health issue, you are going to walk out with one," she said.
"I was not a habitual law breaker.
"I had been in trouble with weed but I was not a criminal per se," she said.
But then 10 years ago, she was convicted for supplying Class A drugs.
"Things went on in prison that had a huge impact on me," she recalled.
"There was a lot of intimidation. You become a non-entity. You lose all your life skills."
She said that "lots and lots" of inmates had mental health problems.
"I'd say probably 80%, had a mental health issue. Depression is huge. Every prisoner I knew was depressed.
"People there are very vulnerable.
"You have a lot of people with personality disorders, addiction, aggression, anti-authoritarian attitudes, being loud and difficult to handle."
Tracey said in the first prison she went to, Holloway, in north London, initially those with mental health problems were kept apart from the other prisoners.
"In Holloway, they had their own gym sessions, education and their own time and space," she said.
But then, overcrowding meant many prisoners were dispersed to other prisons around the country.
At one prison where Tracey spent some time, she met up with a woman who she had been with in Holloway Prison.
PRISON MENTAL HEALTH FIGURES
70% of prisoners suffer from two or more mental health disorders
10% of the prison population has a "serious mental health problem"
10% of men and 30% of women have had previous psychiatric admission before prison
Source: Prison Reform Trust
"There was a woman, who when she arrived was put with another woman who was not a mental health sufferer," she said.
"She was vulnerable and was a target. She cut up her face and was taunted more."
Such examples are quite common, the Prison Reform Trust report has found.
It was based on information from 57 Independent Monitoring Boards (IMB), citizen groups which regularly visit prisons in England and Wales.
Peter Selby, IMB president, said: "(In prison) you have people with mental health issues who are being dealt with by staff who are not trained for the purpose.
"You soon realise that most people in prison are sad rather than bad."
He said diverting prisoners who did not represent a threat into treatment rather than custody would cut reoffending.
Six years on from her release, Tracey now campaigns to improve the treatment of those serving time who have mental health problems.
She still receives help from mental health professionals, but tried to commit suicide two years after leaving prison as she could not initially get counselling help.
And while she thinks it is important that convicted inmates accept responsibility for their actions, she wants people to understand those with mental health issues need help.
"People who know better than us should help us to cope with life.
"Prison is the wrong place for people with mental health issues."
"I know that I made a mistake at one point and went to prison.
"I have paid for my mistake and I would like to use my experience to do what a lot of other people can't do, and voice what goes on and influence some sort of change."