I was in this exact situation not too long ago. The girlfriend I had prior to the one I have now dated some guy for a year and a half. She broke up with him to go out with me, but never got over him. He was her first real boyfriend and her "first." She NEVER got over him, and we had dated for 9 months. I finally said I had had enough of her **** and showed her the door because no woman (especially at our age) is worth that much trouble. Nobody treated her as well as I did and she was unable to see what she had right in front of her. That seems to be the case with you as well. I would give up because she will cause you nothing but heart ache. She clearly is still attacthed to him and you need to tell her that it's either him or you. Make her choose. Be prepared for the worst. Move on.
You never know who you're going to run into anyway. I broke up with my gf on a tuesday night. By Friday I was going out with an even better girl...and couldn't be happier with the outcome

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Oh...and navyboy made a good point in his last post.