Enforcement of Child Support

My ex-husband and I have an agreement that hasn’t been followed now for about 5 years. I didn’t know enough at the time we made the agreement, to include extra-curricular expenses and medical/dental, etc. He stopped paying child support for our two children 3 years ago. We now have the kids 50/50, and he uses that as justification for not paying. He makes 2-3 times as much as I do, but hides that somehow and claims he makes much less. He has a spouse who he shares a house with and another child. He claims taxes for our son, and I claim for our daughter. This happened a couple years ago as he insisted that would be fair. We have always split any sport/activity costs for our children 50/50. Last year, I faced extreme financial difficulty, and without being able to make rent, had to move into a basement living space at my parent’s house as it was all I could afford. He is extremely intimidating, and has bullied me to the point I feel I am about to have a breakdown. I can’t keep up with the financial stresses he is putting onto me. He tries to shame me for my situation.
I don’t know if our original agreement would still be enforceable? Am I able to ask for support? I am extremely down and need to find out what my options are. I need help!