Bad finance contract

I bought my first car from this guys and they didn't even negotiate for a good interest rate at all. <br> <br> Firstly<br> I got a rate of 18% (17.817%) which I thought I'd be able to pay because the instalment wasn't that bad since the car is a cheap Polo vivo.<br> <br> I tried to call the bank so that I can get them to drop the interest rate a little bit, but they said it was too late, that I should have called them before I signed the contract. <br> <br> Secondly<br> This guy linked the interest rate with the prime lending rate ,(which I realized on my account profile that my next installment has increased) and we all know that it will go up and up and up..... Also tried to call the bank to change this, \I should have asked for this to change this before I signed the contract"they say.<br> <br> I understand that this is my fault anyway"