What is the hardest piece of feedback you have received, and what did you do with it?

Short answer

Choose feedback about your effect on other people rather than a skills gap, quote it close to how it was said, describe the reaction you actually had before the acceptance, and show a specific behavioural change that the person who gave it could confirm.

Keep self awareness explicit in the story because the interviewer is testing the behaviour, not only the outcome. A useful self awareness point should make the answer more testable, not merely longer. Tie self awareness back to the scenario so the interviewer can see why it matters here.