Hey hausisse, I can really relate to what you've said in this thread. I've always thought of it as a self-defense mechanism. Like, I'd start seeing a guy and at first I would be very totally interested and attracted. But then that insecurity would creep in (even if I didn't notice it for what it was) and I'd find little things to pick at to make the guy "not right for me". Then I'd lose all interest/attraction and stop seeing the guy.
If he made it past the first few dates without that happening, then around the 2-3 month mark, when I started to develop "more than just like" feelings, this would happen again. I would find things to pick at, issues that I couldn't accept in a partner, and then I'd break things off. The truth was that I was afraid of being vulnerable, of letting someone in, of trusting, and then getting hurt again.
When I met my current boyfriend this didn't happen... and I don't know that I can properly explain. It was like I knew that I could let this guy in... there were times when I noticed that little voice in my brain saying "oh this is a problem" or "I don't like that", but when I actually sat down and weighed these "issues" or felt for validity, I found that it was just me trying to protect myself, and not actual issues with the him. So when this happened I would just talk myself off the ledge, so to speak.
I think part of it is being able to take that step back and really evaluate whether the issues you're finding with these guys are valid, or if you're just protecting yourself. And also, even if an issue is valid, how big of a deal is it to you really? Is it actually a dealbreaker? Keep in mind that no person is ever going to be 100% perfect. We all have flaws and we all make mistakes. Part of it, too, could be that you just haven't found the right guy yet, the one who you feel that you want to let go and let him in!