Try to focus on other things. Find something you are passionate about that doesn't involve your crush. Work on your social life and focus on going out more with friends. Also be sure to remind yourself daily that your crush is a human being with flaws too. He's not perfect and might not have made you happy in the long run. There's someone better out there for you.
Man, do I know this feeling. :-/ Just keep telling yourself that things simply won't work out and that there is no reason for pursuing a relationship that isn't going to happen. Once this reality sinks in, letting go gets easier.