How do i regain friendship with ex?

I'm in a similar boat. Me and my ex were together for 8 years. Obviously we shared alot of memories and our past and stuff. But when we tried to be friends it only made things worse. But i think the fact i still love him and he's still an idiot makes me hate him more and more now that i'm on the friends side of things. I'm not a jealous person. But when i find out he's hanging out with another girl i go crazy. It's just better to walk away with happy feelings about the other person than making things worse and eventually hating them.