Girlfriend broke up with me...

to start dating a close friend of mine not even 2 days after the breakup.

I never abused her. We had a good relationship. I put time and effort into her.

Then suddenly she became distant. We broke up. /story

I just need advice on how to deal. My goal is to be at peace with it; I know that this action on both of their parts makes them not worthy of staying in my life, but I can't stop obsessing about it. Throwing them out of my life will be a staple, sure, but the whole logic behind doing such a thing just blows my mind. Any psych majors here to help me out?

Jam it back in, in the dark.