My first bit of advice to you is to remind you that there are plenty of fish in the sea....some people forget that.

My second is that some people have a hard time getting over serious relationships. A lot of people do actually. If this girl had a kid with that guy, they must have some pretty deep roots. She may be just as confused as you but she doesn't know what to do so she got back with the guy she had, what she felt, was the deepest connection to her. It is hard to win a girl over when she is in that state, especially if you don't completley understand the situation.

On the other hand, she may have never really loved you the same way you loved her and she's moving on. You have to be aware of that. The greatest thing you can do for anything you love is to set it free.