How do I get over him?

Okay, so I really like this guy. But he's made it clear that he doesn't share the same feelings. I've tried everything, crushing on new guys, hating him, everything. I just can't get him out of my head. I get super duper jealous around him and I know he hates that. He almost dated my best friend but his excuse for not dating me was 'I don't want to ruin our friendship'. I don't get it. Why was he so ready to date my skinny, pretty, athletic, smart best friend but not me? I tell people I don't like him anymore, I've even convinced myself a couple times. Then I see him and know I'm kidding myself. I don't know how to get over him. Help me please!