because every so often someone makes the mistake of being nice to me. and i get painfully infatuated. and i then get rejected. and then i feel like shit for a few months, and then it happens again with someone else.

I'm wondering if this is actually the root of the problem. Neither painful infatuation nor feeling like shit are exactly great frames of mind to be in for finding a relationship — and repeated cycles of painful infatuation aren't "normal", if there is such a thing, so perhaps this is what you're doing differently to other people. (I do know what you mean though; it happened to me a few years ago, completely out of the blue, and it was really shocking and weird and uncomfortable and pointless and completely uncontrollable. Once was enough thankyouverymuch.) Having said that, I don't know how to make it not happen, so this comment is perhaps also pointless.