Can’t get over buyer remorse

I bought a used car last month, my dream car. And it’s been all fun but at the same time I have this sense of regret for the price I paid. The paint wasn’t in great condition, nor was the suspension. We agreed on the price of 16,300 and he took the cash and said he’d help me on the sale tax. After depositing my cash to the bank he goes back on his word and writes less than what he’d help with the sales price making it cost 17,500 after tax. My mechanic did a PPI and said it’s all good but a week later repairs came and spent about 2,500 on coolant issues. On the other hand, there’s this other car, same specs, a little lower miles in another state I was considering, which I would’ve went for priced 18,000 that has fixed its well known issue of coolant leaks. I kept wondering if I made the wrong choice and everyday it crosses my mind. How can I go over this feeling of dread? It bothers me everyday and I know I can’t change anything but it’s so upsetting.