The world suddenly feels empty...and I need it to not feel that way. How?

I'm into someone and I really need to not be into this person. How do I make myself lose interest? I've tried the cutting off all communication thing and the ignoring thing, but I can't stop my mind from dazing off fantasizing about things. It's annoying, it's freaking me out and it's making me feel weak. How do I stop it!!!???