Most Helpful Guy
Stop talking to this girl. When girls do this kind of stuff it's to gather everything that is "missing" so she can have a complete existence. What I mean by that is she is missing all sorts of things in her life and is using you to fill those gaps. Drama is one of those things that women want and crave, it's exciting to them.
While this drama is going on she will try to do everything to place blame on you even when you did nothing wrong in the first place. It's what they do. You add fuel to the fire when you get defensive and bring truth and logic to the picture, they never want to hear that. Think about this. She is having drama with you, and going to her boyfriend to have sex with. All her "needs" are being fulfilled.
She had already moved on without you with another guy. That should tell you everything you need to know. By talking to her you are telling her that you still care and might even have feelings for her, and she is using that to her advantage. She is playing games, but you are allowing this to happen to you by talking to her.
Cut the cord. STOP chasing and STOP communication with this girl and move on TOTALLY. Erase her from every social site and delete her from your life. Don't forgive her because she isn't sorry for what she did to you in the first place. She did it, enjoyed it, and secretly laughed her ass off while she did it. You are not "friends" and you never wanted to be.
If you keep this going it's the same as a girl that's in a abusive relationship with a guy but she doesn't leave. You will be saying 2 things. 1: It's not that bad being treated this way (which you should stop complaining). And 2: The main thing in common that you and her will have is that neither you or her have any respect for YOU.