It can feel so unfair! We work so hard trying to do the best we can, many times we've made concessions, gone without, been more than fair too them/for them but still... they act bad, "get away" with it or get a slap on the wrist etc. I know I got the short end of the stick when my AXH and I separated but he still acted as though I'd taken him to the cleaners and tried to give me even less. It feels unfair because it is. It sucks.
You have a lot on your plate SaveHer. It's no wonder you are feeling overwhelmed. I hope you have a support system in place so that you get a chance for some self care. You need some down time to catch your breath and charge your batteries with all that you have going on.
He doesn't pay now, but he wants to pay less... what? Does he think if he has a lesser amount to pay, he will and then he can start seeing her again? It won't be that easy after four years of both physical and monetary neglect. He is probably just trying to make you miserable by dragging this crap out. Try not to buy into it. Fill out the papers as best you can and be done with them. I know its a p.i.t.a. but once you've done it , it's behind you. Best case scenario they review it, see he hasn't been paying and this all comes around, bites HIM in the butt and you start getting a few bucks to help care for your daughter.
Try not to waste too much of your time thinking about him, I know it's hard, but it's not worth it. Hang in there. *hugs*