Ex can't decide on his own????

My ex and I have twin sons together and he's in a relationship with this woman for over a year. They started dating right around the time we broke up. Anyway I hate her. She's always trying to make it seem like they're the happy family and she's already got three kids. It pisses me off that he doesn't seem to be able to make any decisions without her. I know it's because of her that he took me to court to get joint custody of our kids. For example we just went to mediation and I got everything I wanted.....to be the primary parent and he can have visitation Friday at noon to Monday at noon just like we been doing then he goes home to her and suddenly changed his mind. He's going back on what he said in mediation and is back on to getting joint legal and physical custody and is refusing the days we've been doing saying it's not fair to him to have to them every weekend and I get the weekends free. He's come up with a different schedule where it will be totally 50/50 and I know that bitch was behind all of it. How can I get him to see that WE should be making decisions about the boys, not her?!?!😡😡. We was together 5 years and she turns up and I don't even know him anymore. It just hurts and makes me angry. I wish I had never met him or had kids with him but I love my babies. Just wish he wasn't their sperm donor. I wish I could just keep my kids home away from that bitch that ruined my family. 
Update:  I'm not jealous. I wouldn't take him back if he came back on his hands and knees this very second. The only good thing about this woman is that she's got money. How would you feel if the love of your life basically left you for money?  As far as I'm concerned all she's good for is buying shit for my kids which I know she is because his ass hadn't had a job since Jan 2015 and now claims he's in school. 
And HELLO!.. I use the days they with him to WORk not party!