I think once you've experienced that it's very very hard to shake the need to keep an eye on things and spend carefully.
My ex is not from a wealthy family but has never experienced this kind of poverty. His family always had enough money for treats after bills etc were paid, he joined the army at 17 which while the pay at entry isn't great it's good for a 17 year old and they get subsidised accommodation and food, free uniform inc shoes/boots so when they get their pay packet it's basically just spends
When we first married (we didn't live together first as we weren't living in same county) he drove me nuts with his lax attitude to spending and he did actually leave us short one month and his response was "we'll just use the overdraft"
Turns out he didn't know that overdrafts cost! He knew nothing about how banks worked, interest rates, he had a credit card he didn't know if he didn't pay it off in full monthly he was charged interest and a late fee on etc he was clueless. His parents are lovely but they really failed him on this score.
I ended up sitting down with him with paper bank statements (this was over 25 years ago) and literally with a highlighter pen showing him he'd paid hundreds in overdraft costs in the year prior to our wedding. Did same with his credit card statements (which he opened but apparently didn't read!). He also tried to make out I was the spendthrift because I liked going into town of a Saturday with a friend, she drove we didn't and we'd get any shopping we needed (clothes but also household items like lightbulbs or Tupperware type things) then we'd have a McDonald's and then do the supermarket shop (she'd be getting hers too) and she'd drop me home. Again I was able to show him on the bank statements that I wasn't spending nearly as much as he was on rugby tickets, computer games, nights out etc
We negotiated a budget which was also a new concept to him, which included saving some and having a set amount for personal expenditure.
Occasionally he'd moan that it was too strict, but then I'd remind him he was the one that wanted to save for a holiday, or wanted a rugby season ticket or whatever.
In the course of the budget conversation he asked me why I was so stressed about the money because as he saw it, there wasn't a problem as he had such a secure job. I explained how hard I'd had it at times before we met and indeed even when we were first together which he wasn't aware of. I also pointed out that while on paper his job was secure it also had risks - he was due to be deployed shortly after the budget discussion to a very volatile part of the world. His last deployment one of his unit had not returned, he'd died. He hated discussing such things - which I get - but it's necessary too. I had to bug the life out of him to do a will when we had dd.