What do I do?

After 9 months of NC and 19 since the exN and I broke up, why do I still have obsessive thoughts about him? I mean when I have to occupy myself with something really interesting he's absent but when I want to rest myself after a long day or just relax, here he is again. And I can't resist reliving some moments in my mind only to end upset again and sometimes hurt.
How does this go away? I have to focus on me? I do! But these intrusive thoughts are a pain in the ass and the whole situation is absurd...
After thinking for a while about it I realized that I might insist doing that only to reassure myself that I was not crazy, unstable and a total mess like he said I was. Perhaps I have not yet restored faith in myself.
But I also realize that some ubnormal situations cannot be explained with rational thinking no matter how hard I try.
Someone told me that in order to forget I must forgive. I'm not there yet. I can probably forgive myself one day for getting into this mess but I can't forgive the cruelty with which he treated me.
I don' t know....maybe I ask too much for me right now or I'm thinking too much, which isn't good by the way.
I'm afraid I've lost my peace of mind forever...