A behavioral therapist might help, you will need to take it one step at a time, the point is, you keep thinking so highly of this girl yet the feeling isn't mutual, you need to fix that. Telling yourself over and over until you're convinced that she's just like any female friend you've had is a start. You have to hit things at the core, you have to remind yourself that she's just a human girl just like anyone else, what's so special about her that you can't find in another girl? Why should you care? Is it because you feel you owe her some form of debt? You don't owe her nothing and she owes you nothing, the sooner you realize that she isn't the penultimate being in your life the better. Life is about finer things, think optimistically, you should be good enough to find another, spend an extended amount of time away from her if after 3 months you haven't seen her but still it's a problem you may have an obsession which is never good. I mean lets say she did like you even with that other guy, would she or any other woman hang around someone so depressed in body and soul? Of course not, that's why you need to think positively for your sake not hers. Look up the "optmist's creed" and read it to yourself everytime you feel down, you'll gain results it's called in psychology, the self-fulfilling prophecy