Ella Purnell could feel it in her bones that something really bad was about to happen. She was hyperventilating and completely at a loss as to what was wrong. Her mum suggested that it was probably nothing to worry about, and said she might have drunk too much coffee. 

In fact, she was having her first panic attack aged just 13.

“I couldn’t identify what the feeling was,” Purnell recalls. “I felt worried and scared but I didn’t know why.” 

Purnell - who had recently filmed Never Let Me Go, playing a younger version of Keira Knightly - feels the British education system let her down, by not providing the tools she desperately needed to help her understand and cope.

“I felt low and down, but thought you could only have ‘depression’ if something really bad had happened in your life,” she says. “It hadn’t, so I thought ‘I can’t have that’. It’s is a big scary word and it didn’t feel like something I was allowed to have, because we never talked about it.” 

Unable to unpick her feelings and unsure how best to help herself, Purnell’s anxiety worsened and between the ages of 14 and 15, she began self-harming. 

To look at her now, you would never guess that this outwardly successful and striking young woman had struggled with her mental health. The 22-year-old has a CV that would see many an actor go green: as well as Never Let Me Go (she still counts Knightly as a mentor), she has appeared in films such as Maleficent with Angelina Jolie and, earlier this year, played Hester Argyll in the BBC adaptation of Agatha Christie’s Ordeal By Innocence. Indeed, 2018 has been something of a success story for the London-born actor, who also snagged her first leading role: in American drama series Sweetbitter, based on the bestselling novel by Stephanie Danler about a twenty-something finding her feet in New York. Oh, and she became an ambassador for Chanel.  

All this has given her the security and confidence to open up about her depression.

Indeed, we are talking today in the offices of YoungMinds - one of three charities the Telegraph is supporting in its Christmas appeal this year. YoungMinds estimates that three children in every British classroom have a mental health condition, such as anxiety or depression, and that half of all problems have manifested by the age of 14. Its research has found 36 per cent of 16 to 25-year-olds have self-harmed at some point in their lives, and 60 per cent of teachers have taught a child who they believe is self-harming.

Whenever a teenage Purnell felt numb or overwhelmed, she would physically hurt herself, cutting her upper arms deep enough to leave scars, in order to relieve the emotional pain. 

“It was very much like an addiction,” she says, flashing her large brown eyes, which are framed with thick liner.

But it didn’t stop the panic attacks, which plagued her a couple of times per week at their peak. Purnell kept the problem to herself, hiding the marks on her arms by wearing long sleeved t-shirts, or covering them with makeup. 

“Self-harm has one of the biggest stigmas around it, which is that you do it for attention,” she says. “It drives me crazy – absolutely not. I tried to hide mine consistently.” 

But she couldn’t keep the emotional and physical marks hidden forever, especially from her mum, who took her out of school one day and asked her to be honest about what was going on. Purnell wasn’t ready to stop self-harming and didn’t want to seek professional help, but says it was “nice knowing that I had a support system and people that did care about me, because I felt quite isolated at the time”. Her mum was “unbelievably supportive” and told her not to hide things out of fear that she would cause others to worry. 

“She was great; she struck a really good balance of being caring and letting me know she was there, without being too pushy and forceful, which would have just freaked me out,” Purnell recalls. “We still talk about things all the time, she checks in on me and she’s the first person I call if something’s going wrong. She’s one of the few people that can really make a difference with one sentence or a hug.”

Although she has been in the public eye for a decade, Purnell considers herself “not a very open person”. She has a close family and a small group of friends with whom she shares everything, but keeps her personal life separate from her acting. 

“First and foremost I’m a human, a daughter, a friend and a sister,” she says. “I put my relationship with myself above my work.” 

This has made it difficult for her to open up about self-harming, especially because she felt “people didn’t really know how to talk to me or treat me, and it made them uncomfortable”. 

That said, after she told her mum, she felt like a weight had been lifted and her impulse to hurt herself slowly reduced until, eventually, she could deal with her feelings by speaking about them. 

This summer, Purnell spoke publicly about self-harming for the first time, which inspired her to become an ambassador for YoungMinds. 

“You feel like it’s a weird secret you’re holding in, but it’s really not. It’s just a part of you and it shouldn’t be this big ‘coming out’,” she says. “I never thought I would have the courage to talk about it publicly because it’s a very private thing, it’s very personal and vulnerable.”

She continues, “It was really scary, but it’s been so lovely reading messages from people saying, ‘You’ve encouraged me to speak out, to tell someone’. Then I started thinking, ‘this is my thing’.” 

One reason Purnell thinks she is finally confident enough to open up about her mental health is because this is the “first year that I’ve actually taken the necessary steps to start looking after my mind”. She now has a therapist and has put in place coping mechanisms that help reduce her anxiety. 

Although she doesn’t have a clear idea of what triggers her attacks – it can be noise, loneliness, tiredness and stress – she can ease them by lighting a candle, taking a bath, reading a book or calling her counsellor, who has “absolutely changed my life.”

She also also learnt to congratulate herself for every achievement - however big or small - be it getting out of bed in the morning or passing her driving test. “It’s the best feeling in the world that builds you up a little bit and keeps you going,” she explains.   

Another help being mindful of her social media usage. “I don’t scroll [aimlessly] anymore, because you always end up seeing something that changes your mood,” she says. “Really put yourself first. It’s a hard thing to do when you’re growing up.” 

Although self-harm has left enduring scars, Purnell accepts them as part of her history. “I don’t really see them anymore,” she says. “They’re just part of my body, they tell a story like this stupid cartilage piercing I tried to get when I was 14, or this hole in my belly button.”  

They are also a sign that she will always need to work on her mental health, even though she now knows that it’s “OK not to be OK”.

“You have to work really hard, constantly, at it,” she continues. “There’s always the possibility that you could relapse. Self-harm is always going to be something I think about, because for a long time that was how I coped with pain.”

She pauses before adding, “Start that communication; check in with your friends and follow some feel-good Instagram people. The first thing that changed my life was telling someone”. 

Young Minds is one of the Telegraph’s three chosen charities for our 2018 Christmas appeal. For details of how to donate call 0151 284 1927, or visit telegraph.ctdonate.org