Your ex never made you happy and you finally ended the relationship. Even if he dates others he has not changed as a person. You are still grieving what he never provided you with in your relationship. It’s normal to grieve. However, don’t fall into the psychological trap of wanting him to suffer and feel loss. By doing that you are hanging on to what led you to want a divorce in the first place.

I think therapy can help you learn to distance and begin to focus on your own happiness. You were stuck for a long time so it’s going to take you time to learn how to focus on yourself.