Jesy Nelson "mentally bullied" and "starved" herself for years in Little Mix.
The 29-year-old singer - who quit the band in December following a break - admitted she had "wasted years" of her life because she thought being "skinny" would "please other people" and make herself happier.
She shared a photo of herself on Instagram lying on the floor after a workout, with her ribs visible, and wrote: "I just saw this picture pop up on my feed and it made me feel so sad.
"On the day this picture was taken, I remember going to the gym with my manager and I stood in the mirror and I said, ‘god I’m so fat I just want my legs to be like this’ and I pulled my skin around to try and create a thigh gap because I genuinely believed that being skinny would make me happier.
"And even when I got to a size 8 from starving myself, I still wasn’t happy. I mentally bullied myself everyday to try to please other people."
Jesy urged her fans to follow her lead and be kinder to themselves and stop worrying about what other people think.
She added: "It’s taken me 10 years to realise that I’m the only person I need to please.
"As humans we find it so easy to pay other people compliments but yet we find it so hard to be kind to ourselves because it’s so much easier to pick ourselves apart than to find the things we love about ourselves.
"So if you’re someone who is feeling like I was, please don’t waste years of you’re life seeking approval from others, seek approval from yourself.
"Be your number one fan. You are the one in control of your happiness and you are the only one of you in this world and that’s pretty f***ing powerful!"
When Jesy quit the 'Wings' group in December, she explained the "pressure" of being in a girl band was too much for her.
She said: "The truth is, recently being in the band has really taken a toll on my mental health.
"I find the constant pressure of being in a girl group and living up to expectations very hard."