I just don't understand how we have that spare but dp says we can't afford to buy a car
Well, ask him. Money not spent on expenses is not 'spare', it is savings. If you are frittering away spare money on little things, that you think means you can afford credit for a car, then get that under control first. Otherwise you will still have the spending habit And have the credit payment, which is bad news.
Savings is an expense. As someone who also came from behind like you, that was a hard concept for me to finally wrap my head around. When you never have enough cash for the endless list of things that are constantly breaking (because they were the cheap version, or used) or being outgrown, or that you are making something stand in for, or just not having, then you are used to spending Anything 'spare' in that manner. If you know that any attempt to save for something nice will end up being used on something necessary, you get in the habit of cheap pleasures like takeaway and scrounging around for somehow when something breaks. But now your income is stable enough that savings can actually grow until big enough to use, and then actually be replaced. But it only happens if you spend your spare on the savings account.
But it is easy to go on treating everything in that constant re-purchase mode, and always see unspent cash as spendable and not yet accept that not spending is a thing you can and must do.
So, that's how my financial learning curve went. Talk to your dp though: what does HE mean? What picture and plan does he have in his head, and hope you can learn? It is possible that his picture and plan is just his own vague money fears and issues and unlike my dh, he isnt just ahead of you on the learning curve after growing up poor. But you wont find out either of those things from us.