Without trust in a relationship, it can be hard to move forward. In order to save your relationship, you have to work on getting that trust back. If it can't be done, then you shouldn't continue the relationship.
If your girlfriend has cheated on you before, she should be an open book for you now. She should hide nothing. Why? Because those with nothing to hide, hide nothing. If she gets defensive about what she is doing, then this is not a good sign.
How long ago did she cheat? If it's recent, she should be doing things to reassure you and work on your relationship to get your trust back. If it was not recent, has she done anything recently to raise your suspicions?
Have you really forgiven her? Forgiving means you have put it behind you and aren't going to throw it in her face every time she makes you angry. But that doesn't mean that you have to keep your eyes closed to the fact that it could happen again. The best predictor of future behavior, after all, is past behavior. But if she hasn't done anything wrong since then, and has been putting forth her best efforts in your relationship, perhaps it's time to give her some trust again.
As for what you should do in the short term, decide if your relationship is worth saving and if you are both willing to do what it takes to work on it and get the trust back. What are you worried about happening at the party? Are you able to attend? If not, why not? What would have to happen for you to feel comfortable with her going? Are your expectations reasonable? These are all things that should be talked about.
As for the long term, if you decide to stay in this relationship, I would suggest seeking counseling for both of you.
I hope this helps. Good luck to you!