Demi weighed 9 1/2 stone when she started dieting - and was a size 8 on top and a size 10-12 in trousers.
But, at her lowest, the then 16-year-old's clothes were hanging off her and she weighed just 7st.
She said: “When I first lost weight, I received compliments and was getting noticed a little bit more.
“I think I started to get addicted to the compliments, the weight loss and looking at the numbers on the scale.
“I was getting more and more pulled into it... I started taking photos and checking my body all the time.
“My eating disorder was based on a number on a scale. I would weigh myself 30 times a day.”