Barry Greenstien's book talks about this several times. He has some good advice he uses.

One was when he recognizes he's playing bad, he gets up and goes to the bathroom and washes his face. He looks in the mirror and talks to himself...he says "If you don't start playing better, you have to quit." And if he doesn't play better, he actually does quit.

Another one was when he's on tilt and he needs to quit, he tells himself "This is what differs you from them. When you're stuck, you can quit. When they're stuck, they won't be able to quit, and that's when you'll get your revenge."

Those quotes are paraphrased a bit, but you get the idea. It works for him, maybe it can help you.