Anger help please

What are some ways to not hold anger towards someone? Me and my bf have been together for 4 years my daughter is 1 month and when I first found out I was pregnant he cheated on me. Now 10months later I catch my self thinking about it idk why I do. Is that my post partum depression???? I'm a ftm also. It just secretly I have anger towards him. Idk how to help my anger. And we never talk about it so I have to keep my thoughts to my self.