I'm really bitter towards my ex's girlfriend, how should I deal with it?

[Kiwi Kitty]

I was with my ex boyfriend for 7 months, and towards the end he cheated on me with this girl, (who he is with now, and has been for a few months.) Before this we were totally okay with each other, not exactly friends, but we were on good terms. After all of that stuff happened I was really cruel to her, and I said mean things about her and fought with her a lot. Now there is this thing at my school where a small group of girls meets and talks about their issues/ The thing is, she's in the only group I can be in, and I have a LOT of issues that I wouldn't want a lot of people knowing about. I can totally give her a chance, and trust her with this information, but I'm worried that she won't feel the same way. I'm with someone else now and totally over my ex boyfriend, but I asked her if she was okay with me joining the group (over Facebook messaging) and she wants to talk to me about it in person today... I feel like I would really like to be on good terms with her again, and maybe get to know her a little better, how can I try to convince her to give me a chance?