I'm bitter towards my girlfriend?

Well, I've been with this girl for a while now. I used to feel pionate love for her, and our relationship was strong. But, she got restless and ran off with my best friend. I was devastated. It hurt so ******* bad and was definitely the worst experience of my life. Not to mention, it put a huge riff between me and my best friend of many years. Anyway, their relationship quickly deteriorated and she came back to me, and I took her back (as well as repaired my relationship with my friend). So, at first I was overjoyed to have her back. But lately, I've noticed that I'm beginning to resent her. I'm constantly moody around her, and downright cold at times. I guess it can all be traced back to the fact that I never really forgave her for what she did to me. Part of me still loves her, but I can't seem to get past what she did. So I'm not sure if I should break up with her, or try to forgive. Advice?