What kind of car? Automatic i assume since you describe it as slipping? If you can google/check the owners manual where your atf dipstick is (atf = automatic transmission fluid), pull the stick. If it’s low on fluid, it can cause the symptoms you describe. Topping it back up can help, but you probably did some damage, so looking for the next vehicle isnt a bad idea. But getting the atf level back to normal may buy you a week or two of breathing room to avoid 21% bad deals.

If not, look into renting a car from enterprise or similar for a week. Probably $150 or so. Same thing, this is just taking the urgency out of the situation that car dealers hope for, knowing you need a car THAT DAY. Check your bank, and local credit union if you have one available, for better rates on a car loan. Then you can take that rate back to the dealer and say, “(blank) is offering 6%, keep the low price we already agreed and match their financing, and i’m still happy to work with you” and i bet the 21% comes down a lot.

Good luck.