All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Taking a loan with Riba (interest and/or usury) is prohibited. So, you are obliged to repent to Allaah. Indeed, if you sincerely repent to Allaah, He will accept your repentance. For more about the conditions of sincere repentance, refer to Fatwa 86527.
As for your cousin, if he able to help your repay your loan, he should do so. However, if by hastening to repay the loan, the interests will be waved off from you, then you are obliged to do so, otherwise you are not obliged to hasten to repay the installments and return the loan if this does not lead to getting rid of the Riba.
For more benefit on the ruling of avoiding taxes, please refer to Fataawa 84560 and 85356.
Allaah Knows best.