Girlfriend of 2 years flirting with her ex?

My college girlfriend of 2 years has been flirting with her ex boyfriend from home on Facebook messages, and saw him behind my back while she was there for the holidays.

Over the summer I caught the same thing going on, confronted her about it, nearly broke up, but in the end forgave her because I love her. I told her if she did it again we would break up. She was very remorseful and told the guy that she regretted what she had said to him and didn't want to speak to him any more.

Now it happened again, 6 months later. I broke up with her, but it's still kind of in the air. I love her with all my heart and I want to forgive her and have things go back to normal, but I don't know if it's healthy. I need to grow a spine and stand behind what I told her this summer -- if she does it again, we break up.

She's clearly remorseful (again) and swears up and down that she has never cheated on me (physically, I guess she means). She has been crying for hours and her mom is coming out to pick her up, she's a mess.

I just don't get why she would start talking to him/seeing him again if she knew this is where it would lead. Especially after 2 years of a very serious relationship.
Girlfriend of 2 years flirting with her ex?
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