Abandonment Issue Advice?

I'm a girl who can't hold a healthy relationship. I've assumed it's because my dad left and my first few boyfriends weren't there for me. Infact my whole life guys have let the women in my life down. There is this really great guy who I like that says he loves me but everytime we get close I mess it up. I don't know what to do because being together hurts him but being apart hurts us both. If that isn't enough I think I might like my best guy friend who also likes me. Someone please tell me they can help me because I have no clue what to do!