How do you stop caring about someone?

So I dated and became best friends with this guy, it lasted for years. Put simply, he decided that I wasn't good enough and he became someone else, someone that most people don't like too much now a days.

I still care about him, a lot and I really shouldn't. He never explained anything to me, he never apologized to me, he just turned into a cocky ass hole and wrote everyone off, even his best friend.

But then I seen him again. Whenever I see him (and he's alone), he'll match me -- a wave if I wave, a smile if I smile. Every once and awhile he'll talk to my best friend. I know that he does still care a little bit because you can not erase 6 years together.

But then when I see him, with his immature group of "friends," I just remember what he's become. He walks around like he's the sh*t. I actually feel sorry for him!

I do not want to get back together with him -- EVER but yet I still care so much about him and to be honest, he doesn't deserve it. I should not be there for someone who would not be there for me.

And on top of it, every time I see his girlfriend or any of her friends, they either start whispering about me, laugh at me, or give me dirty looks. They are all dumb sluts and they can be immature if they want, but I don't like the idea that maybe he talks badly about me.

I've done just about everything. I've deleted his number from my phone and I haven't actually talked to him since May. I thought maybe we could be Facebook friends, but he denied. (SInce then I've deleted my Facebook). And I honestly feel like myself again, like I'm happy but he still has a piece of me and I want to get it back!