Not every interview goes your way. The quicker you realize this fact the better. Some of the most qualified people just are not good at getting up after rejection. The quicker you get up and keep going after one rejection the faster you can pursue your next interview.
Even if this job is a lost cause, you never know when you might meet this interviewer again, either for a different job at the same company or for another company that the interviewer has moved to.
Here are 3 steps you can take to try and recover from your mistake.
1. Think back.
Analyze what happened and exactly what you think gave the interviewer a bad impression. Stick to definite feedback that you received during the interview, not just a "feeling" that something you said wasn't received well. No point in apologizing for things the interviewer may not even have noticed.
For example, if the interviewer expressed surprise that you knew so little about the company, you will have to address...