How should I handle this on/off relationship and now cheating?

I dated this girl off and on for 3 1/2 years. It wasn't the easiest of relationship to be involved in we both had made mistakes and we have broken off multiple times. She would mainly be the one to break it off. She would say "you never spend enough time with me" or "you're not meeting my needs". The Killer was I spent as much time with her that I could when I wasn't at work or spending time with my kid etc. but it was never enough. We'd have a massive blow out and then a month to 2 months later she'd come back and feed me a line like "you're the total package" B/S.

Well about a year ago she meets another dude when we were broken up and starts to date him. He ends up cheating on her and she comes back to me. Well apparently they remained friends after the break up and I find out yesterday that she had sex with him a week ago (while she is dating me).

This is the first time I've been cheated on and I'm not sure what to do. She paints this dude as being a total loser. No job, lives at home, no money, pothead. But they have a "connection" and similar interests (WTF). This is killing me, she made no effort to apologize, and is the one ignoring me.

What is your take on this? How do I walk this one off? This is some of the most Bipolar B/S I have ever been involved in!
How should I handle this on/off relationship and now cheating?
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