Advice!

In the past, a friend of mine has belittled me, embarrassed me, and just flat out treated me poorly! Despite the way she has made me feel, I try really hard to be happy for her, but sometimes it's hard because I have felt like she hasn't always been a good friend to me. I don't know why it's hard for me to be happy for her and I really want to! Any advice on why I might feel this way and how to overcome it and be happy for all her successes?!