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The 29-year-old, who quit girl band Little Mix last year, shared a photo from a few years previously. The image sees the star lying down at the gym, with her ribs protruding.
Jesy captioned the post: "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."
She continued: "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 your 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!"
Fans praised Jesy for speaking so candidly about how she struggled to accept herself, with one writing: "I love you Jesy Nelson, for being such a a powerful, talented and an incredible woman. You're tremendously inspiring and words can't explain how much we love you. Don't you ever doubt of yourself, you're beautiful inside and out."
Another said: "The most beautiful and inspiring person I know. I am so proud of you...you are more special than you will ever know."
A third added: "You are phenomenal."
Jesy has previously documented her struggles with her mental health after being horrifically trolled and abused online when Little Mix was competing in The X Factor in 2011.
She discussed the extent the trolling had on her wellbeing in BBC Three documentary Odd One Out, where she even discussed attempting to take her own life at her lowest point.
Jesy also explained that her wellbeing was the main reason why she decided to quit Little Mix.
Describing her nine years with Little Mix as "the most incredible time of my life", Jesy thanked fans for making her feel "like the luckiest girl in the world".
"The truth is recently being in the band has really taken a toll on my mental health," she wrote. "I find the constant pressure of being in a girl group and living up to expectations very hard."
If you have been affected by anything in this article, please contact the Samaritans on their 24 hour hotline on 116 123, or follow this link for further information.
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