Create a ritual, something you do each time you get an especially vicious or unmerited rejection.
I usually pace around the house picking things up off the floor (gaining some measure of control and organization). I like to throw in a few sets of push-ups as I go, a physical action to work out the stress.
Music is good for this phase. Something angry. Eminem works. He’s pissed off enough to match the mood, plus he can actually write (nothing’s worse than hearing a string of published cliches after your original work was dismantled by an pathetically unprofessional editor).
Wash the dishes, hands in the therapeutic warm water, rapping along with Eminem. When he yells, you yell too. Rhyme in unison. Drop alliterations and assonance.
Then get out your laptop or typewriter. Get ready to go. Write a few notes to yourself.
Now, shut the music off.
And write for two hours straight. Don’t stop for anything. This is key because you can’t prove them wrong if you don’t produce. And writers write. That’s what we do.