The unfortunate events of July are irrelevant. I can understand that it RUINS your mood and outlook towards things, and I'm sorry to hear about all that, but really...they're separate issues.

That aside, you need to leave that girl. Alright, she told you...but the fact that she didn't want you to know earlier is no good. Leave that girl...if she can't stay faithful to you, fuck it. How are you supposed to feel like you've got someone special when she's making out with every next guy she goes to a movie with. I mean, that's what I'd do...no "forgive and forget" with garbage like that. If you've lost that much trust...then it's probably not worth it.

How long have you two been together anyway?