Accident with rented car (PRIVAT/ without insurance “declared”)

A good year ago (July 2014) I borrowed a car from my girlfriend because I started my training and I came to work badly through the rail replacement traffic. I drove the car with her stepfather over 1 hour so that he could see for himself who he rented the car (old FORD KA automatik ) Tüv to OKT 14. The car was no longer the best and should also be scrapped after the end of the tüvs. We then also worked on the insurance for 2 weeks. Everything went well until I didn’t drive for 5 minutes alone. I drove around a turn and swooped… once completely over. TOTAL SCHADEN! He decided not to let it be clarified about the insurance. The only incurred costs were those of the “cleaning” of the oil track (approx. 450€ ). For the time being we had agreed that I would pay it by instalment (50€/mtl. if possible. otherwise less) (because Azubi with 311€/mtl can not pay everything once). As soon as I only participate in leisure activities that cost a bit (open-air swimming pool, café, …), I get a message that I should rather pay the amount off. I am tired and simply no longer interested in it! That the friendship is probably there too. Do I have to cover the costs because it wasn’t my decision to let go of the insurance? Do I have to cover the costs completely? Greetings and thanks in advance for every answer