How do I get over it?

So I have always had friends, but, rarely are they a close friend. I find it especially hard for this to happen with guys because I'm not really into sport and you can't flirt with them.

However, last year, I became friends with this guy. We were quite good friends I guess, we would usually tell each other about parties and stuff. He was kind of like a wing man.

A few months ago, he decided to hate me and was making fun of me behind my back (I know this because he does it openly) and I think has made people hate me. I then confronted him and he shrugged me off, so I thought "F**K you then".

We now hate each other unsparingly and it caused our friendship group to split in two. And whenever we see each other we both get extremely angry but we try not to cause a scene. But he just P***ES THE HELL OUT OF ME, EVERY TIME. Even though we have both graduated now, I can't avoid him because we have a lot of mutual friends. How do I not get so angry whenever I see him?
Updates
+1 y
I think he might have started hating me because we stopped hanging out as much, in school and outside of, and my curiosity got to me. However I stopped this behavior and didn't bother him for a few weeks and we were as we were, THEN he started hating me.
How do I get over it?
Post Opinion