Should I repay the lender the full amount, even though I have already repaid the loan amount and more through usurious installments, and how can I repent from this prohibited transaction?

Sincere repentance is obligatory for explicit usurious transactions, as they are among the gravest of sins. Repentance is achieved by regretting having taken the loan and resolving never to return to such an act, along with returning the borrowed money without the usurious increase, which is forbidden to be returned if it is possible to refrain from doing so.