I Hate My Ex Husband

My ex husband will have nothing to do with his three younger children who live with me. I've tried everything to get him to see them but he really isn't interested. Our eldest child lives with him and the woman he ran off with, who he married. I've always spent time with my eldest son and see him every possible opportunity I can. The trouble is my ex keeps trying to push me out of our boys life. If there are appointments we need to attend say for our sons college he makes sure the appointments are when I can't go cause of looking after are younger kids. Then he takes his wife to these appointments instead. She even introduced herself as his mum the other day. I've always been close to all my kids and I never wanted my boy to live with my ex but I lived in the country and he wanted to be near his mates in the town. The thing is now I'm just so sad cause it feels like they are trying to replace me in his life.