How do I stop feeling like a bad person?

My mom hated me as a kid. When I tried to do something nice she said I was being fake. There wasn’t a week that went by that she didn’t tell me I was selfish etc and she would shun me anytime I did something she didn’t like–wouldn’t speak to me or even look at me, sometimes for days. The upshot of this is that now, no matter what I tell myself, at heart I still feel like there is something wrong with me, that I am fundamentally a bad person. It doesn’t even feel like a belief–it feels like a horrible fact I go out of my way to avoid looking at. Remember the show “Dexter”? I only saw it a few times, but sometimes I feel like that, as if I’m just inherently evil and the good things I do are icing on top of something that is rotten beneath. A few things that have happened this summer feel like proof that my mom was right, and I feel sick over them. I want to stop feeling like this and I don’t even know where to begin.