How can I get him out of my mind?

i really love him but we are not together.. we never were. he doesn't like me..he is all day on my mind and that's drives me crazy.. I cant continue like that anymore. it hurts me, and it really hurts me to see him with a girl. what can I do to survive this.? I mean I am all the time upset about that..i am sad and nothing feels ok. I cant stand it anymore.. I really nead to rellax.. to be happy again.. I need something to get him out of my mind. and I dont know how