Why Can't I Let It Not Affect Me?

So of all the dates my ex could have chosen, he chose November 15 to start dating someone new. November 15 was the day he started dating me.

I feel overwhelmingly sad... and am mad at myself for feeling that. His new girlfriend is the one who is getting involved with a guy who, really, is incapable of keeping up a deep conversation and of falling in love and loving even when it isn't convenient. She should be the one who is sad, not me.

I think I am more mad that he has someone and I don't and that makes me feel like I am not as good... I know it's not true, but I still feel it. I know I need to separate "his stuff" from "my stuff" and not let his affect me.