Help me find out why I can't let go of bad relationships?

I am 27 yrs old and finally in a relationship with a great man. But for some reason I can't forget an ex who was a liar, a manipulator, and left me 3 times. I am wondering if it comes from my relationship or lack of one with my father? I've only met him twice although he knew about me my entire life. I met him both times at 15. Most of my relationships have been with abusive, cheating, loser types...like him or what I've heard about him. The ex that has really affected me is the only guy who has actually walked away from me and made me look like a fool 3 times yet I still wonder about him and what might have been? I started counseling today, but I was wondering if I could get any clues as to why I'm letting this affect my current relationship. He wants me to figure it out before we continue together.