How can I stop being obsessed with my ex?

We had a rocky relationship almost from the start. Huge fights and he cheated. He broke up 1.5 years ago and since then we've had a little contact now and then and spend the night together sometimes.

A few months ago things started to get better and we started seeing each other more often. A couple of weeks ago he told me he didn't want to continue seeing me and I found out (through facebook stalking) that he is with the girl he cheated on me with back then. They seem to be in a relationship now.

I'm devastated and hurt. But I'm also obsessed. I can't stop looking at his (or her) social media. We are not friends on any site anymore but I still look at their accounts as a non-friend/follower. It's basically all I do all day. I've tried to block them everywhere but soon after I go and unblock them or even create new accounts so I can see them again. I've tried dating others ever since things ended 1.5 years ago but I don't get any feelings for anyone.

I can't stop thinking about him and her. Even when I do other things or are with friends, family or other guys I think about it and check their profiles... It's not healthy behaviour and it's only hurting me more to see how happy they are and what a great time they have on their holiday now. But I don't know how to stop!
How can I stop being obsessed with my ex?
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