More than 50,000 children and young people turned to Childline last year because of a serious mental health problems, figures show.
New data from the National Society for the Prevention of Cruelty to Children (NSPCC) shows one in six Childline counselling sessions are now about serious mental health issues.
Some 50,819 children and young people in 2015/16 received counselling for a serious mental health issue, a rise of 8% over the past four years.
Those aged 12 to 15 made up a third of all the sessions, with girls almost seven times more likely to seek help than boys.
Childline saw a 36% rise over four years in youngsters needing help for depression and other disorders, while there was also a rise in the number of children and young people 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."
Childline founder Dame Esther Rantzen said: "It's striking how many more children seem to be suffering serious mental health problems today than when we launched Childline 30 years ago.
"So many desperately unhappy children seem to be suffering suicidal thoughts, self-harming, becoming anxious and depressed, with many of them turning to Childline because no other support is available."
Dame Esther said children's mental health services are already "terribly overstretched, meaning that young people aren't able to access the professional help they need".
She said: "We believe there needs to be far more emphasis on providing help for these children as they struggle to deal with what are very serious illnesses."
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.
The figures were released at the start of Children's Mental Health Week.