I've only ever had it happen once, and it was awful, but i sat down afterwards and considered my options, and my first conclusion was this "i know I'm a very good guitar player (Sorry for the narcissism) and vocally i can stay in a tune, and making music, this is something i really love being able to do. so i can either fold now and sit in my home studio pretending I'm enjoying myself or stop being an ass and get on with it. the second conclusion was that when i gig, I'm doing something that a high percentage of the audience either cannot or will not do and which they can only dream about. and my final conclusion is that once in a while no matter how much i rehearse, every now and again at least one of my gigs is gonna be a bit of car crash and that is something i have to accept, but the more gigs i have done the better my performance gets and i have learned something magical and that's stage craft, which is half the battle