I've had/have a similar problem. Honestly, I still think about my ex that I really cared about even now. But I've found that if you let them, things go away by themselves, as long as they aren't around for a new surge of nostalgia.

For example, I used to still wear around a lot of things that my ex gave me, since they were nice. She gave me a watch, some random clothes, and a few other things. I used to think I would get over that stuff eventually, and just be able to wear the clothes. Never really got any better, and I just threw them out. I remember that they were there, but now I don't have to see them every time I open up my drawers. The more of her that I've "deleted" from my system, the less I remember about her or the relationship in general, since I'm not seeing it all of the time to remember it. Eventually, hopefully I'll forget about the whole thing and finally move on completely. At least that's the plan. Facebook is still a bit weird since I haven't deleted it and occasionally check up on her profile, but it's been less and less as I think about her less and less.

And for thinking "What might have been?" This one I find much easier than the random pangs of nostalgia from everything I see that has memories of her associated. Whenever I think about it, I think of why it didn't work out, why it wouldn't have worked out and remind myself that memories become fonder over time, and we tend to forget the bad things about the relationship as a whole. I tend to think about what the consequences of that action would be or would have been, and come to the conclusion that it's not going to happen, and fantasizing about it is useless. Busy myself up to forget about it. Dwelling on everything just makes it worse, if I find myself doing that, I try to do something productive.