You ask yourself if you like the idea/image of being "so damaged" you "can't trust anyone". Some people get something out of that kind of thing, even if it's nothing more "keeping the betrayer's evil alive" (maybe so other people will know how "bad" that person was). Or, sometimes people just need to live as victims. They sometimes see things in black-and-white: If you're "the betraying type" you're the one with upper hand. If you're not the betraying type, you're the victim (and they equate victims with being the nice person, with strong people being potential betrayers).
The idea that just because one person is, say, a bank robber, it doesn't mean everyone is a bank robber shouldn't be that hard for anyone's mind to accept. So, I think the thing is for a person to ask himself if he's getting something about being "permanently damaged in the trust department", and then ask what it is he gets out of it. One thought is that if we allow ourselves to move on and get over something, we don't get to see what the betrayer did as quite as big and permanently damaging as if we "will never get over it". So, maybe those are things to ask yourself if they apply at all to you. Other than that, my thing is this: Why let someone who has betrayed me take away more from than s/he already has? I get what I need out of feeling that I'm bigger and stronger than "some jerk's" betrayal. I like "having fight" rather than "feeling beaten and fragile" when it comes to how I want a betrayal to affect me.