Why do I want him back?

I broke up with my ex almost a month ago because he cheated on me and lied. I don't regret my decision because it was the right thing to do, but lately I can't stop thinking about him and my feelings for him are growing more than ever. It's crazy because when I left him I was already in the process of losing feelings for him so to let him go was easy but now I want him back even though he doesn't deserve me and i'm not sure if our relationship is worth fixing. I don't know what to do. I want him back but the thought of what he did helps me to not take him back. I'm having mixed feelings. Any advice???