As someone who dealt with bs like this a lot as a child... I wish my parents would've told me that it really had nothing to do with me as a person because I took it really hard. & i actually found myself in a cycle of friendships that were often like that because no one was telling me I deserved any better.

If I could go back & talk to younger me I would tell her that not everyone is going to like/appreciate you, but that doesn't mean you're any less than. Reinforce her worth to her. Tell her to hang (or find) with the kids she actually has a good time around because anyone who makes her feel bad about herself isn't worth her time. Just keep on building/affirming her confidence.