1. The interview is not over just because you made a weak first impression.
You can overcome a weak first impression by quickly recovering your confidence and work on making a good impression throughout the interview. You need to temporarily pretend it did not happen and mush forward. Convince yourself you will deal with the slip-up toward the conclusion of the discussion as you "end on a high note." If you make a good LAST impression, the start of the interview will be forgotten (as long as you didn't set the table on fire or ask who the ugly kids were in the picture on the interviewer's desk).