Squash your defensive instinct. Don't react until you take it home and ponder. I want to see if there's truth to the criticism. One method for handling criticism is to depersonalize it. It's painful to discover our mistakes, but it's even more painful to go through life repeating those same mistakes.
Criticism is like a needle.
It may sting. But the pain passes - and the results will benefit you for a lifetime. If you recognize that the purpose of criticism is to wake you up and get you on the right path, then you'll be grateful to your critic. If your critic has only said it to put you down, this makes you bigger than him. Don't react on the spot.