How to get over an ex?

I have an ex who is a textbook sociopath. Yet, I still can't shake off the rest of my feelings for him. It has been almost five months since we have split. I have a new boyfriend and he has had two new girlfriends already... Cheated on them both. Like I said.. Sociopath. He did the same to me. He's very narcissistic and charming, manipulative and a great liar.. But I've caught him cheating and we all have. So with knowing all of this info, why the heck do I still care about his life? I have a boyfriend who I care about so much. I stay busy. Yet whenever I find out my ex is single or talking to a new girl, I just wish I could butt in and call him out on his bull crap even though it isn't my problem anymore. Even though I know he'll never change. I just wish I didn't care.