This is a really good post and very sound advice.
However, I still think you (OP) are mischaracterising the problem. It's really not about 'his' car, and 'your' flat.
You both are financially 'illiterate' (putting in quotes so that it doesn't sound like I'm being insulting - I really just mean neither of you seem to understand the basics like, how much household income you have, where it actually goes to, how much is discretionary spending and how much is essential, and is the balance fair, in terms of income / payments etc.
I agree, that it's mostly likely that all your income is going on child-related spending and you have no sense of control or autonomy as your DH is paying the big bills, but most likely, also has a lot more disposable income.
You need to get counselling / advice around this issue - both, I think. Counselling to unpick the relationship dynamic, and advice e.g. via reputable financial advisor or even your bank.
It's quite shocking to see the attitude you both have to finances and debt, when you have a family.
(And specifically on the flat - I don't think selling it without a wider review makes sense BUT equally, I don't think it's as simple as 'it's yours' and so you get to decide to hold onto it. You need to rationalise your spending, and address the debt and that WILL involve reducing spending, increasing income, and reviewing assets, your flat included).