Stage Fright - I touched on this in your other thread; your recent recital. But, this thread has brought up some new thoughts. Personally, my bouts with stage fright came from being unprepared, either musically (not practiced enough before playing in a public place), or mentally (not knowing what to expect from a particular venue.
At 14, I was asked to join a rock band, (partly because I had some cool bass gear, my dad had a van, and we could practice in my garage), and we got hired for a block dance, way before we had polished the songs. It was pretty scary, but the cool factor, of just being in a band, that people, mostly kids our own age, would come to see, outweighed the stage fright. Musically, we sounded pretty sad. But, to our surprise, the "audience" didn't seem to care. They danced, sang along with some tunes, asked for songs we didn't know, and some we did know, and at the end of the night, we got some compliments, felt pretty good about the whole thing, and got paid. It was a lesson.
Depending on the venue, the music you play is only a part of the dynamic. Again, growing up, there was an annual festival, games, food, and music, and every year a band that played mostly Italian music was the big attraction. The crowd loved these guys. The bass player sang most of the tunes, but he was a terrible bass player, lots of mistakes. But, NOBODY CARED. Except the few musicians in the crowd, and they didn't matter. Those guys got paid a lot, and they were there every year. Why? Because it wasn't just about the way they played. It was the energy they could produce, the song choices they made, and they were in the right venue. Could that band play a concert and be well received? No. Could they play a party? Yes. Were they "Impostors"?
I'm not sure I subscribe to the 'Impostor Syndrome". Musicians do whatever it takes to do the job. A lot of people I've played with over the years had their own way of dealing with the venue, or stage fright, drugs, alcohol, whatever. But a key thing to realize, as a performing musician, is to decide if the gig is right for you. The Blues Brothers took a gig playing behind chicken wire. And it's a good thing that chicken wire was there when the crowd started throwing stuff. Know what to expect from your gig. Realize that the audience has a different perspective (and maybe can't tell what's going on musically), and the performer has the edge. After all, they're the ones on the stage.