Many people find that "one special girl" and settle down with her. No problems so far. But then, they assume that since they've settled down, they can drop all the attitudes and habits that the woman was attracted to in the first place. Take a look at yourself and how you acted around her. I'd be willing to bet that you changed your behaviors once you settled down with her. And that change affected her level of attraction for you.
Make no mistake, the game is never-ending. Even within a relationship, you need to keep displaying attractive behaviors and avoiding the unattractive ones, or things will end up exactly like you've outlined for us here.
P.S.
About your attitude regarding the UG:
It didn't work with her. Who cares, she's a UG! Not only are you now aware of the faulty beliefs that have caused problems for you before, you're free to go out and attract other, more attractive women!
Sounds like a win-win situation to me.