How do I get over my ex boyfriend?

my fiance and i broke up a month ago, we were inseperable for 2 years, but now he doesnt want to be with me anymore. his new group of friends got him to hook up with a girl the same day we broke up. we had our whole lives planned out together.the worst part is that i see him everyday. we go to the same school, church, and were bot on the track team. he brings his new girl to track because she picks him up and takes him home. ive already hooked up with someone else, but am now repulsed by the new guy bcs he was just a rebound that didnt work and is too clingy. i need time to myself but i just cant stop thinking about him. any andvice? step by step plan?