Children with mental health problems are not receiving treatment until they are in crisis and sometimes suicidal, doctors have told the BBC.
A letter leaked to Panorama reveals at least one area's child and adolescent mental health service (Camhs) to be rationing care.
Leading psychiatrist Jon Goldin described the service as "not fit for purpose".
The government said it was investing an extra £1.4bn in child mental health.
An 'awful' two years
Jess, 14, first began feeling anxious three years ago, shortly after she went to secondary school.
"I felt like everyone was judging me all the time… you start to get just horrible, horrible feelings of more and more and more worthlessness."
Her mum, Claire, tried to get help from Camhs, but was refused and Jess's health got worse. Claire began sleeping next to her daughter to try to keep her safe.
"I was woken by Jess saying, 'I love you mummy'... I knew that was alarm bells and I ran out of my bedroom door [and] she was standing with the noose around her neck."
Jess was taken to A&E for treatment and was finally seen by a Camhs crisis team. But by this time her health was very bad. Her mother took videos of her having psychotic episodes to show doctors, to prove how ill her daughter was.
Jess tries to describe how these episodes felt, saying it was like "all your negative thoughts and all your awful things that have happened over the last week all just collapsing into one hour, almost like putting you in a bottle and shaking you up and then letting you out again".
Eventually, Jess was detained at Chalk Hill, a specialist child mental health hospital in Sussex, for her own safety. She spent nine months as an inpatient, and says she was assaulted by a staff member.
"I was having a psychotic episode so I was like running on the floor, he grabbed my legs, pulled me back, did that and shook me upside down."
Sussex Partnership NHS Foundation Trust, which runs the hospital, says concerns about treatment are taken "extremely seriously" and investigated "as a matter of urgency".
Earlier this year, Jess was released and is now recovering back at home. She was told by the hospital that she had autism.
"I feel like if it had been diagnosed ages ago I probably would never even have had this journey. I would've just gone, 'OK, I need to do some therapy for Asperger's because it'll help me control my thoughts.'"
Her mum agrees. "We were crying out for help and support for a very, very long time and she shouldn't have deteriorated to this level."
Children 'having to attempt suicide'
The number of under-18s seeking help for mental health problems is rising, with at least one in 10 children thought to have a diagnosable condition.
Demand for Camhs services is up and some areas are struggling to meet demand.
A leaked letter from Waltham Forest service in London, obtained by Panorama, states that for six months earlier this year it raised its threshold for treatment, so that only the most seriously ill children - such as those with psychosis - were treated.
North East London Foundation Trust, which runs Waltham Forest Camhs, says it has now lowered the threshold again, saying: "We do not underestimate the level of challenge we are facing."
Panorama heard similar stories from staff working in other services. Only one in four children with a mental health condition currently receives treatment.
One woman interviewed worked in Camhs for decades but recently left because of her concerns about the quality of care she was able to offer.
"My experience in the past eight to 10 years has been a real deterioration in children and adolescents' ability to access services - the thresholds have changed considerably."
The Association of Child Psychotherapists says it has heard of cases of children having to make multiple suicide attempts in order access care.
Nick Waggett told Panorama: "We do hear stories of children and young people having to have attempted suicide on a number of occasions actually before they are seen within the service.
"The problem is that then they're very ill and it actually becomes increasingly difficult to offer them an effective treatment."
The government says it is investing an extra £1.4bn in child mental health services, and "transforming" the service "so that 70,000 more children a year have access to specialist mental health care by 2020-21".
But Labour's MP Luciana Berger, who sits on the health and social care committee, believes NHS services are spending the extra money elsewhere.
"In too many cases that money just isn't reaching the front line. I believe urgently we need to see those monies protected and ring-fenced."
According to the Care Quality Commission, evidence suggests the number of children visiting A&E for mental health treatment has more than doubled since 2010. Many services are failing to meet NHS guidelines for an out-of-hours crisis service.
A Freedom of Information request from BBC Panorama has revealed that at least 1.5 million under-18s in England live in areas without 24/7 child mental health crisis care.
Dr Jon Goldin, of the Royal College of Psychiatrists, said the system was unable to meet the demand: "I have colleagues working all over the country who are… doing excellent work, the trouble is the services are very stretched. We're not meeting the need so in that sense it's not fit for purpose."
BBC Panorama, Kids in Crisis is on Monday 24 September at 20:30 BST on BBC One.