hi Tom
It is fairly easy to get caught in some sort of self-awareness hell on stage, at least for me when it happened I was on some level nervous about personal scrutiny even though I was rehearsed etc. and felt confident about the music. It still comes down to some form of stage fright.
I found that a few things on stage helped:
Focus on a few individual members of the audience and sing/play to them. This works in several ways. If you are focusing truly on them, you wont be focusing on yourself, plus it makes for a more engaging performance for the audience.
Get into the meaning of the song you are performing. An emotional connection between you and the song can help pull you out of self-analysis. Yet again this also makes for a better performance from the audience perspective.
Interact with other band members during songs.
Both of the above are active ways to avoid that hyper-self-awareness. They take a little practice to do effectively.
The reason, I decided, was that I was so rehearsed and comfortable playing that performance was sort of automatic... leaving my mind space to wander into self-analysis, and the stage is not the place for that! It happened when I was getting back into performance after a break and had somehow forgotten to take the show to the audience. ie I had remembered to prepare musically for the show, but had forgotten about other aspects of performance. A simple reminder, a mini eureka moment and a little practice and the problem went away.
Focusing on conveying the song to individuals and interacting with the audience gives your mind something constructive to do.
Although you may be rehearsed etc the notion of being judged is higher at an open mic night.
One other point to make would be that not all anxiety is necessarily obvious or apparent. We might not be aware of just how nervous we are before a performance. You might not feel that way on the surface and when the initial symptoms are noticed it comes as quite a surprise. That surprise part I think makes us more vigilent in looking for that issue again in the future, each time it happens we look for it more the next, and eventually it becomes a self-fulfilling prophecy. By breaking that cycle of thought by focusing on something/someone else it reduces the ability of the problem to take over.
I hope this helps
Cheers
John