"To get to my school I had to cross a railway bridge and I would just stand there and think 'I could just jump off'.

"It felt overwhelming and like I was alone. I just felt so, so low, I couldn't see a way out."

Calleigh, now 18, was self-harming when she was 11 and first thought about taking her own life when she was 13.

"I didn't want to tell my parents because I thought they would be worried and disappointed or not understand."

Calleigh's story comes as the charity Childline says it has carried out its highest number of counselling sessions with young people having suicidal thoughts and feelings.

But the helpline says it can answer only three out of every four calls and urgently needs more volunteers.

In 2016-17, a total of 22,456 sessions were given to children in the UK thinking about taking their own life - up from 19,481 the year before.

More than 2,000 contacts were with young people who had already taken steps to end their lives, such as writing a note, giving meaningful items away or even planning their death.

For Calleigh, a perfectionist nature, coupled with extensive pressure from a high-achieving school, meant she sank to a very low point.

But making contact with Childline proved to be a lifeline - literally.

Childline counselling 2016-2017 15% increase in sessions about suicidal thoughts since last year 22,456 sessions about suicidal thoughts

72% of those who had this counselling were girls

13,746 sessions about anxiety

295,202 counselling sessions in total Jupiter Images

"I was able to talk to someone, they were able to say, 'OK, what's making you feel like this? Take one step at a time' and they're just so supportive.

"I'd call them when I wanted to self-harm. I would message them - they've got amazing messageboards - it's like a forum but it's safe, you're not going to get bad messages and there are so many other people that are feeling the same way."

What should you do if you feel like this?

Calleigh says it's important to seek help - and to remember that there doesn't always have to be a reason for feeling at breaking point.

"I'd say you definitely can't bottle it up, because that makes it so much worse - just talk to someone, even if it's online.

"Talking to someone who's not in the problem, who you know is not going to judge you or worry - because my main problem was 'I don't want my parents to worry, there's so much stress' - and just knowing that you're not alone and that it's not your fault.

"And there doesn't have to be a reason, it doesn't have to be that you were bullied, it doesn't have to be pressure, it could just be you feel numb because you feel numb."

What does the future hold?

Childline founder and president Dame Esther Rantzen says it's vital to find out why so many young people are feeling so desperate.

"When Childline launched over 30 years ago, I remember children usually felt suicidal because they were being hurt by someone.

"Now young people tell us they are overwhelmed by mental health issues taking them to the brink of suicide.

Media playback is unsupported on your device Media caption Former Childline user Jamie: 'Just having someone to talk to'

"We must discover why so many of our young people feel so isolated they turn to Childline, because they believe no one else cares about them."

Calleigh wants to see much more done to raise the profile of mental health issues.

"I strongly feel that teachers, parents and support staff need to know more about mental health and they need to look out for the signs."

In the meantime, Dame Esther is urging members of the public to come forward as volunteer counsellors for Childline.

"Anyone who can lend a few hours to this vital service could end up saving a child's life."

Help can be found by ringing the Childline helpline on 0800 1111 or via the Childline website.