Oh, dear.
Let's hope your finance company does better than a house loan mortgage company (Washington Mutual) I dealt with once. Paid more than the required payments -- they stuck it all into a separate account, deemed the payments late, and charged late fees for each monthly payment. Phoned them, they then entered correcting entries for each changed line item. But then an auditor later didn't understand, and re-charged the original penalties for each affected payment. This happened THREE times, the account statement ran on for many pages. They just couldn't deal with overpayments! Finally pulled the loan from them -- and they went bankrupt shortly thereafter. Can you imagine the hassle it would have been trying to explain that to the receiver company on the bankrupcy?
You can set this up fine as a spreadsheet, with interest charges monthly, payments credited to interest and principal variously monthly, and principal carried forward. Why haven't you done that-- Cheers, --Mr. d.