I know that the two of you have been in a relationship for a long time, and you want to try to work things out. But she honestly seems to be giving you more greif than she's worth. She lied to you when she cheated on you, she lied to you when she said she wouldn't do it again. Come on.. writing that letter to him is a form of cheating too. Maybe she isn't sure what she wants. Maybe she is in dire need of professional help. Is she really worth it to you? If you truly want to give it a go with this other girl, then you should. But you need to break it off with your current girlfriend before you do it. Do you remember how you felt when you found out she was cheating on you? You don't want to stoop down to her level. Good luck. You seem like a very nice guy... the world needs more people like you.