The soulful 25 year-old, who performed her first single, Nothing's Real But Love on the show last week, told how she grew up in care and foster homes because of her mum's illness.
"Growing up, we didn't have anything," Rebecca told The Sun. "My mum wasn't well so I want in three care homes, then foster homes, before me and my little brother went back to her. I was passed from pillar to post."
That childhood must feel like a million light years ago now. Thanks to her steely determination, the former secretary and single mum of two finished second to Matt Cardle in last year's X Factor final. And next week she releases her first album, Heaven, which has been raved about by critics and tipped to outsell Rihanna.
It's a dream come true for the Liverpool-born singer, made all the more poignant because of her tough upbringing.
"When I was 16 and my mum had moved to London, we were living in this flat," she said. "We were really struggling to do things like put money in the gas meter.
"Constantly having to think about money is not nice. People used to say, 'Being rich doesn't make you happy'. And I'd think, 'I've got no electricity, nothing - tell that to my empty fridge'."
That poverty made Rebecca determined to escape and make a better life for herself.
"I did have determination when I was little. I just saw things around me and thought, 'I'm going to get out of this, this is not going to be my life, this is not going to be my mother's life'," she explained.
"I worked in a shop when I was 14 and started going to singing lessons in the evening at this posh prep school where all the rich kids went. Obviously I couldn't afford to go to the school during the day so I went to my £20-a-day job and just went to these classes anyway.
"I'd turn up at this posh school on the bus and everyone would be looking at me with their noses in the air as I handed over some cash while all the mums were giving in cheques.
"I used to come out and their dads would be picking them up in these Rolls-Royces and stuff."
Rebecca left school and got a job as a beauty therapist, but when she was 17 she became pregnant with her first child, Lillie. At 19, she was pregnant again, with her son, Karl.
"People told me my life was over," she said. "I didn't believe it though. I ended up training to be a legal secretary. I just wanted something stable for the kids while continuing with the music. Maybe it's the Scouse in me, you just get on with it."