I can recommend a book called "How to be happy" by J. Alexander. It's a sort of self-help book for children, but not at all cheesy. Presented in a 'cool', humourous style, it goes through some basic CBT so they can change their own negative thinking patterns into positive ones.
Are you confident because that would affect her. Dies she do a hobby? Guides drama or whatever. I find with dd 11 if she's not feeling great about school she still has her outside friends at her clubs. My dd is skinny but beautifull and a talented dancer. Other girls can be very unkind about her "spine sticking out" I use humour a lot to help her come up with answers or reasons why the fat girl person might say theses things. It's a funny age isn't it? Hope it gets better.
It sounds like she might be good candidate for CBT, which you might be able to help her do some of the tricks of without having to get professional counselling. Just getting her to stop and question herself when she's upset, she would need to work it thru outside her head, on paper maybe:
"I feel Y I feel Y because I believe X. What is the evidence that X is true? Should I still believe X?"
You're right, of course. Often she'll say "You only say that cos you're my mum."
She doesn't seek external validation; we've just noticed that she really only believes praise if it's someone impartial who gives it.
She is hard on herself - she takes after her dad! We praise her for eight or nine out of ten for a spelling test, but she's not happy unless she gets ten.
Being our only child, she gets all of our attention, which must be pretty intense for her. With no sibling to compare herself against, I think she compares herself against us, so she's always going to find herself lacking!
DD (10.10) has never had much self-confidence. She doesn't think she's good at anything, doesn't believe us when we praise her.
She has always had good reports at school, no problems there. She doesn't often put herself forward, but then again neither do DH or I.
Her lack of confidence affects her willingness to stick at anything. She's never read a book for pleasure because she's "rubbish at reading", and she's on the brink of giving up the flute because she "can't do it".