I am not sure what I can do to help, I've been in exactly one relationship, and it seems to be working, so I can tell you what we do, but I can't promise that the others are wrong or that mine will work for you.
We are a single income family with four homeschooled kids. My income is OK by itself, but after paying for all of the homeschool stuff as well as the same taxes and such that we'd pay if using public school, along with three years of a very expensive private school for the first two kids, our savings account is gone and the incoming isn't that wonderful. Hence, we're usually pretty strapped.
We both have the same values for kids and our family, so we agree to buy more expensive food as a rule, not an exception, to give an example.
Tunatruck was bought as a 'restore while drive' vehicle as a proof of concept that I could restore a vehicle from the ground up without major support and without huge cash outlays. Since that started, a bunch of kids happened. The wife understands that the truck now has to be finished, because it's still worth $500 scrap no matter how many hours and dollars I put in until it's basically finished.
That said, she's not limiting me. We agree that we don't have money to spend on things. All of our money is all of our money. She makes some extra money from violin lessons and a few other things and stashes it away, but it's still our money. I get gift cards from work occasionally, but it's our money, not mine (except when I am buying her a present with them, and then it's mine).
So I get like $50-$150 per month, usually, to spend on the truck. If we have more, and I need stuff, she's OK. If we have less,a nd I have to spend $0, I understand.
I lease a Leaf for DD duty. I can't touch it. Our other vehicle is a minivan. I don't want to touch it. I am going to finish the truck before spending any on any other automotive pursuits. Not sure what they'll be. Lemons is a possibility since I tried to do both for a number of years.
Anyway, good luck. Money sucks. It's really hard to deal with.