How do I get over this anger?

My child’s father was somewhat involved then started dating someone with 3 kids (one close to our child’s age) then suddenly wasn’t involved. At some point even stated he wanted to get his parental rights revoked. Hasn’t seen our child for over 7 months. Recently he has been somewhat responsive and stated he would want a relationship with our child in the future. Yet no effort is being made. I find myself getting this overwhelming anger at them. I don’t want to hold onto this but I don’t know how to get rid of it. They post pictures around the holidays of him holding the baby, he went to his gf nieces birthday instead of his own child’s party at his family’s house. She made it clear she doesn’t want communication with me yet buys my son birthday toys after the fact and is helping with child support paperwork. And now there is no communication with the father for the past few days since.