I used to have problems like this. You have 2 options:
1) Ask her out
2) Forget about her
If you want option #2, just picture her doing the most disgusting things. Picture her sitting on the toilet, grunting, farting, and then wiping her ass. Picture her pulling out a tampon, and there's a root growing on it. Remember, when it all boils down, women are just humans. They just happen to be attractive people. Also, good looking women are everywhere. Look in your local department store, library, fast food resteraunt. They're everywhere.
It's also possible that you're attracted to what you can't have, which is a natural human feature. Everyone wants what they can't have.