Time tends to be the best remedy. As best as you can, focus on other things in your life. Make yourself get out of your house and have fun with friends. More importantly, don't stay in contact with this person, at least until you've moved past your feelings for them. Being friends with someone who doesn't reciprocate your attraction is painful, and you may become angry or resentful when you see them date someone else. In perspective, is it worth your time to dwell upon someone who doesn't think you're compatible enough to be in a relationship with them? If you find yourself thinking of this person, remember one of their poor character traits or an insensitive thing they did to you or someone else.
Avoid them as much as you possibly can. - Write a list of every unkind thing you can remember that they have done to you or someone else. Keep it somewhere near and reread it whenever you start feeling low again. As you feel so intensely for them, you will initially always come up with an excuse for them for every mean thing that they have done. With time, you will realize the extent to which they really aren't as perfect as you have initially made them out to be. Let it all out to a friend and then do not talk about them. At all. Every time you do this you will be reinforcing your memories of them, no doubt idealizing them as you go. Distract yourself. Go out with friends, watch films, force yourself to continue living your life without them. Fake it til you make it. Eventually you will realize how much fun you can have without them. Unrequited love can often shatter your confidence. So write yourself another list, with all your positive attributes and aspects of your character that your are proud of. (e.g. 'I am kind, funny, a good listener, always there for other people, etc...') End it with a positive affirmation (e.g. 'and I will get over him/her') Repeat this list to yourself regularly. You must remind yourself how great you are, how idiotic someone would be to reject you and how lucky someone else will be to have you as their partner!