How do I stay sane in a relationship?

I'm a thinker. and a worrier. I've been in a relationship for 6 months and we practically live together most of the time. I guess im pretty needy in a self destructive way. I love her to pieces but i guess I see the worst qualities and see her as weak at times (basically, i've got a whiter that white past and she's made mistakes).. I know not to judge her for it, but it gives me concerns. I guess i just pick up on anything that can be seen negatively and make the worst case scenario out of it. Its frustrating because this is the only problem we really have and at times suspicion, doubt and lack of trust almost overthrow everything we have (and that would be such a shame!). Basically, I'd like to know how to NOT think, or to stop worrying... hope this makes sense :s