Wrong calculations on a retail installment contract!!! I'd already signed off!?

Here's the deal. I don't know what happened and how I didn't notice this at the time...either way, my question(s) are...

If I have already signed off on a retail installment contract and then I come home to realize that the dollar amounts were added up incorrectly...(in favor of the creditor!)....leaving me paying MORE!..is the contract still valid? Is there anything that I can do? Can this be corrected? Rescinded by way of MISTAKE!
I honestly feel this was done intentionally by the creditor to leach more money outta me, and I refuse to be stuck with paying an amount not really DUE!

take your paper work back to the creditor, and show them the mistake. if you need to, try getting an accountant, to figure over payments and any interest that was involved to get a true balance. then demand payment. if its worth it you can file a small claims suit against them, if they don't want to settle, let them know that you will file suit!