“When she stopped singing, I started,” Linda Harrison recalls of the moment when she chose her future. “My grandmother sang to me every Sunday night. This was really special to me, but she died when I was 7. When you lose something you have to fill it with something, and for me it was this dream of being a musician. There’s never been anything else. I doubt there ever will be.”

Harrison is 20 now. She has her first physical single, “Overexposure”, due in September, 1 million YouTube hits, a fashion line and an album recorded when she was 16 to her name, and collaborations and connections from Frankmusik to Perez Hilton. But this seemingly glitzy start is the result of fierce independence and self-belief. She has refused management and publishing deals to pour her own savings into doing things the way she thinks is right. Her upbeat pop songs are rooted in emotional hardship and solid experience gained from Edinburgh to LA’s slums. Vulnerable but with a steely stubbornness, she is a pop star who has paid dues the latest X Factor puppets don’t know exist.

“In my teenage years I got into heavily emotional music, acoustic stuff that I would find online,” she says of her early influences. “And I listened to punk-ska, and Weezer and the Foo Fighters. Then I got into Slipknot…”

Traumatic events around her influenced the music she liked, and secretly and bashfully made. “I had a lot of experience of suicidal friends when I was young,” she says of her early years in Edinburgh. “The first suicidal friend I had was when I was 10, and it just kept on going. That forced me to get more into music. There was so much going on that I had to get it out somehow. I wasn’t talking to my parents or telling my friends, so I needed to write about those things.”

School music classes found her down-graded for gravitating towards pop and modern minimalists such as Philip Glass, not the approved classics. She couldn’t wait to leave. “I found myself really struggling, very depressed for a really long time. I would sit in the back corner and it felt like I was in this bubble, and there were jail doors surrounding me. I was lucky in some ways, my life wasn’t really that hard. But at the time I found it impossible. I had told my parents I couldn’t stand being at school any more. It was absolutely useless to me; I knew what I wanted to do.”

Harrison’s parents let her leave school before she was 16, on condition that she got straight A-grades, and had a back-up plan for her ambitions. She got the grades, and a place at Guildford Academy of Contemporary Music. Already busking daily at the Edinburgh Fringe, she also wrote and recorded her first album. “That album is quite dark,” she says. “It was therapy. I uploaded it to Bebo and MySpace, and a video to YouTube, and people started watching it.”

Harrison moved to Guildford when she was 17, where a teacher made her realise the significance of her YouTube success. “My song “Sunday Morning” at that point had 25,000 views, but I hadn’t ever stopped to think: ‘Shit, 25,000 people are actually watching me, and liking it.’ I found out that the next Black-Eyed Peas single was going to be “Meet Me Halfway”, and covered it before it had been played on the radio, and got over 500,000 views. I realised what the power of YouTube was then.’”

Graduating from Guildford in 2011, Harrison cashed in some savings for a leap of faith in her musical future: a life-changing trip to LA. “I didn’t know anybody there,” she remembers. “It was scary. Getting on the plane, I thought, ‘What have I done?’ I arrived at night, and there were a lot of strange men saying, ‘How you doing? What are you doing tonight?’ I got to my hostel, and there were homeless people everywhere, and two mentally ill homeless people having a fight.”

Some of the sights in this strange new land jarred her. “I started in Venice Beach, where I would say almost 50% of the people I met were homeless and often mentally ill. I found it quite difficult that they’d just been left there, because there’s no NHS. I would travel up to Hollywood, and the contrast was stark. It hasn’t come out in my writing yet, but I think it will.”

Harrison travelled to Austin, Texas’s South By Southwest festival, where she met Ellie Goulding, Linda Perry and Perez Hilton. She had already met the likes of Katy Perry and Noel Gallagher at a party Perry gave that New Year: the musician’s life she had dreamed of now in touching distance. A writing session back in LA with ex-Phantom Planet guitarist Jacques Brautbar changed her music. “We wrote “Supermodel”, which was very tongue-in-cheek. Lyrically it sounds like a pop song Rihanna might even sing. It’s not about sex, but it’s pretty suggestive. I understood pop music then, because when I sing that song on stage, all it is about is the audience having fun, and having that same moment I had with Jacques when we were laughing at every line.”

This completed a change of heart for the one-time Slipknot fan. “When I was 17 I realised I couldn’t write a happy song. I was frustrated because the ballady singer-songwriter, the girl on the piano – that wasn’t who I wanted to be. And so I made a conscious effort to write more upbeat songs. The first ones were angry. Now they’re sarcastic and snipey…”

Harrison’s latest lyrics – “Love Flu”, “Love To Hate You”, “Dirty Beat” – share a theme. “A lot of my songs are about being scared to fall in love, because it can be like an addiction. I certainly have a desire to fall in love, settle down, have children. But there’s a constant grinding. Because the other half of me wants to be on stage and touring the world.”

One song, intercut with Arab Spring footage for its video on YouTube, shows how Harrison hasn’t become a simple pop singer. ““Breakdown” is about government corruption,” she explains. “It started with a strange vision that I had, of hundreds of thousands of people climbing up the streets of Edinburgh towards the old government building, where the Prime Minister was inside cowering, knowing that he’d done wrong. When I saw the images of the Arab Spring it was exactly what I’d imagined, but in a different country. It really affected me.”

A 21st century girl, Harrison sees no need for an album yet. “Singles are what I should be doing right now. Life’s moving at a quicker pace for everyone. But when I do an album, I’m going to make it a film too, a story where one track leads to another and you have to listen to the whole thing.” This old-fashioned commitment to quality extends to her line of T-shirts, which she not only designs and has expensively printed, but adds hand-crafted touches to herself.

Linda Harrison lives in London now. She thinks she knows why she has been so relentless in chasing her artistic objectives, sacrificing every penny she has on a dream.

“When other people I knew were suicidal, it leads you to feel that way as well,” she says. “And any time I felt close to doing anything like that, I thought, ‘No, you’ve got something better to do. You’ve got something to live for.’ I still occasionally think, ‘Don’t give up now. Your 14-year-old self trusted you to do this. You have a responsibility to her.’ When I got accepted into that university, I screamed with excitement: ‘This is it. This is my chance. And I’m not going to waste it.’ And I won’t.”