I confess: I've been playing guitar for over 50 years. Of course, there was a stretch of time when I never took one in hand, but time waits for no one...
Ninety-nine percent of the time I'm performing for myelf: Solo. At home. In private. If I perform in public, any stage fright I might have goes away very quickly, as I get into the pure enjoyment of playing. And having someone else listen (it's better if they're a captive audience, like in a church setting).
However, there's always a part of me that, upon hearing someone else play, says, "Don't you wish you could play as well?"
I had a weird experience today.
I came across a video (I've long said if the Internet existed back when I first started, I'd be a guitar god today), and after watching it, said to myself I'd like to try that. After only a very short time it occurred to me: I already know this! In fact, I was ahead of what the video was teaching. I felt very pleased with myself. I also felt somewhat surprised: Do I have a level of knowledge that I take for granted? Can I actually play better than I often believe? The problem I have with my performances is that I have no one to give me feedback. Last year, when I finally took lessons, I had someone who could hold my feet to the fire (e.g., this scale form begins with your third finger) but I never got a sense of whether my playing was "good" or not.
Self-doubt can be a real downer. I can't begin to describe the joy I get from playing guitar, whether I'm the only audience or not. That doesn't stop me from getting surprised when it turns out I know more than I think I do.