If the criticism is in a letter from an agent or editor, take their advice (if you believe it is correct) and send the query or manuscript back to them. Of course, often no means no, but a few years back a woman showed me a letter from a publisher about her children’s book. The negative nature of the letter had depressed the woman, yet I had always learned that any personal rejection letter was great. The woman took the suggestions, resubmitted the manuscript and went on to happy publication.
What about someone criticizing your work? Someplace in Christopher Vogler’s THE WRITER’S JOURNEY, he talks about absorbing the power of your enemy. I found this to be a helpful piece of advice. In one of my first writing classes, another student made open fun of something I wrote. When I got over my hurt, I acknowledge the truth of what she had said and turned it into a great little scene. Absorb the power of your enemy.