As in everything in life, you always have a choice: quit or continue. This can become painfully clear in the moments before you take the stage.
For me, there's always moment when the fight or flight impulse kicks in. The trick I play on myself is to categorically rule out the flight option long before the gig. Damn it, I paid all that money for new strings, sound gear, transportation, I'm going for it. And, hell, I ain't leaving here until I've gotten my comp drinks from the bar and the only way to get free drinks is to play.
I'm also reminded of the amusing line from years ago: I've suffered for my art. Now it's your turn.
A little swagger, a little bravado and a total disregard for what other people think can go a long way, in music or in several other endeavors.
In other words, tell the critic in your head to shut up for a while.
Rich