How to let go?

My experience with my child’s father wasn’t any mis-treatment while pregnant since he left when I was 8 weeks. Every day I feel like I’ve grown, but it’s still so much anger that hides beneath the surface. The resentment is still there and I want to just be rid of it so bad because I feel like it’s holding me back emotionally. My child is 3 months now and every now and again that man still crosses my mind and I just want to let that hate go and never think of him again. There’s no love and no wanting a relationship between him and my kid or anything, I let him completely remove him self, I just despise him so much for what he did .