when someone else keeps changing their mind

My husband and I are separated. First he said he would pay the full mortgage for 3 months, then half until divorced. For one month he paid it in full. Then he said he would pay only half after that first month until the divorce. Now, he’s not paying the second month because he paid full the first month so he’s using that full payment as a half payment for this month. So, he keeps changing his mind and not keeping his word. Here is my UM and IM on this…
C: he keeps changing the amount he says he will pay
T: oh hell no, this is BS, he can’t do that
A: get a lawyer
R: make it all a fight and lose money anyway by getting a lawyer

New Model
C: he keeps changing the amount he says he will pay
T: wow, he is really struggling
F: compassion
A: let him deal with his own shit
R: take care of myself, find another way to pay bills

So my intentional model is telling me, do not join in his struggle and make this worse than it is. I can pay my own bills, I don’t need his money (but it sure would be nice so I can use my money to focus on what I want to use it on). It’s more the principle of it, the fact that he keeps changing the amount that is my issue. Not the money. So, I have a model on him and how he should behave. I’m on the fence with the thought “I have no control over what he does” or “I can lawyer up and try to force him to contribute”. But my ultimate GOAL is we reconcile as I am not the one who wants a divorce, he is. So, I’m trying not to rock the boat in that respect… but I’m still pissed and thinking this is unfair.