My ex bf hurt me a lot. I broke up with him to see if he would change.He is now dating someone else. I feel really depressed. How do I move forward?

I thought he would change. Instead, he tried to ruin my life (get back at me in every way by turning my friends against me). I loved him a lot so it was very difficult for me. He is now dating various women and doing the same thing he did with me. I only went on one date but I feel bad about it. I just feel depressed even though I know I made the right decision. But how do I stop from feeling bad/missing him - move forward?