Can I change some of the items of my divorce after it's been completed and finalized? Visitation and such?

I would like to go back and amend some of the visitation details of the divorce. My ex is not coming often, he's blowing off the kids and coming when he feels like it then forcing them to spend time with the woman who was his mistress and is now his gf. When they see their dad, they only want to spend time with him and he's pushing her on them, making them very uncomfortable and upset. He tells them he's coming, then will tell them the day before that he isn't coming after all. After two and half months of no visits, he brought her and they were so upset they didn't want to go with him for the visit. Now he's wanting to take them for the weekend, not telling me where he's taking them. It's causing the kids and myself so much stress, every week.