Flynn, it doesn't sound like you weren't with him for a tremendously long time, yet it was probably a pretty intense time, am I right? The bad times were awful, but the good times were probably spectacular. And maybe part of you was drawn to that intensity so that now your "normal" life somehow feels a little flat, a little boring. Many of us here have been in the same boat (myself included), noticing when we looked back at the partners we'd chosen previously that we had a history of choosing the same unhealthy but "exciting" type again and again. Maybe that's not you, but I'm guessing that missing the intensity of that relationship might be part of your continued thoughts about him.
And also, Flynn, sometimes what we're obsessing about is more the "could have been" aspect than anything that really existed. When we look back, we can't help but erase the bad parts of the picture and replace them with our favorite bright colors, painting a scene that never truly was. We mourn the possibility rather than the actual thing. Again, I've done this myself, and even now, 6 years post divorce, I still sometimes do it.
Be gentle with yourself, give yourself time, and as much as possible, when those thoughts return, try to remember realistically what it was like. You will, indeed, heal. It's just that healing often takes forms we don't expect, and it sure doesn't proceed on the schedule we'd like it to follow!