More than 50,000 children and young people called Childline last year suffering from a serious mental health problem.
Data from the National Society for the Prevention of Cruelty to Children (NSPCC) revealed one in six Childline counselling sessions are now about mental health issues.
A total of 50,819 young people received counselling in 2015/16, an increase of 8% over the last four years.
A third of callers with mental health problems were aged 12 to 15 with girls almost seven times more likely to seek help than boys.
The charity saw a 36% rise over four years in young callers needing help for depression and other disorders and a rise in the number of children feeling suicidal.
One 16-year-old girl told Childline: "I want to know what's wrong with me. I have been seeing a counsellor about my mental health issues recently but they just told me to eat and sleep better and forget about the past, which didn't help me at all.
"I wish I could speak to someone who actually cared about me and I felt comfortable talking to."
Another young person said: "I'm struggling to cope with bipolar. One minute, I feel so low, like I'm trapped, and all I want to do is disappear.
"Then suddenly, I feel the complete opposite, and I'm really happy and I start thinking about everything in a really positive light. I feel like I push away everyone that tries to help, I tell them I hate them and blame them for everything. I just feel like I've turned into a monster."
Peter Wanless, chief executive of the NSPCC, said: "It's deeply concerning that so many young people are contacting Childline with a serious mental health problem, with some of this suffering being a direct result of the individual having previously experienced abuse and neglect.
"To ensure the next generation aren't left to deal with a mental health epidemic, there has to be a much more comprehensive network of professional support and treatment in place that is easily accessible for young people who are desperate for help to get their lives back on track."
Founder of the charity, Esther Rantzen said children's mental health services are already "terribly overstretched, meaning that young people aren't able to access the professional help they need".
Childline said early signs of a serious mental health problem in children and young people include becoming withdrawn from friends and family, being tearful or irritable, sudden outbursts, and problems eating or sleeping.