Accident with total damage / insurance

Hello dear community, for a year now I have been in trouble with my former insurer (CosmosDirekt) for my car. Background is an accident with total damage caused by a colleague with my car. He did not have a valid driving license at the time of the accident. I have seen the driver’s license a few months before myself and my colleague also admitted that he has withheld this information from me. Nevertheless, the Cosmos claims that I had dealt with it grossly negligently. That’s not right, because I saw the driver’s license a few months before and I’m not obliged to ask him about the driver’s license every time I let him drive by car. According to CosmosDirekt, now I’m also liable, and should pay the debt over 5000,-. What are my chances of success? Is it worth consulting a lawyer? Greetings!