Ex bf went to whatever extent to hurt me by dating my coworker as well as taking his new date to our anniversary spot. How should I behave?

Even though it’s killingme inside, he publically defamed me and made all his friends and family hate me. I feel like I am being bullied. I broke up with him because he kept breaking up with me and getting back together whenever there was something he didn’t like. I was so in love with him and wanted to marry him so breaking up was extremely difficult for me. On top of the break up he is trying to make me pay for it and go out of his way to hurt me