When I was a kid I was pretty shy (hard to believe now
)...though strangely, I was never bothered by playing music in front of others, and I was doing the piano recital competition thing when I was maybe 10-11 years old in big auditoriums. I wasn't some virtuoso piano player...and I think mostly I would just focus on the task and ignore the audience because I was trying not to fuck up.
I also spent a lot of time playing guitar and piano for fun with friends and in school...so I just sort of rolled with it even during my youth and teens.
When I got older and put together my first band...it was a pretty big deal, I mean it wasn't your usual garage band thing, it was straight to serious gigs.
Even as the band was forming/rehearsing, we already had guaranteed gigs, for good money...mostly because people knew I could play, and that we were taking it seriously, and because we knew it was a real band and we had agreements to fulfill...so we all stepped up and rehearsed a lot, sounded good, and the stage butterflies were gone after that first couple of tunes.
I know it's hard for some folks, no matter how good you play. We had another guitar player in one band, and he could play great during practices...but on stage he would always have anxiety, and when he took a lead, he was never as comfortable as during practices.
After we did a bunch of gigs, he ended up leaving the band because of that ...much to everyone's disappointment. We tried hard to persuade him to stay and said that it would get better...but man, he just couldn't shake the anxiety.
Best advice I could give is to focus on your playing, and try hard to take an "I don't give a fuck" attitude...
...not about the playing, but about the fact that there are all these people listening.
My view was...that even on my shit performance moments, I was on the stage and they weren't...meaning, I had the guts to play in front of them, and just by being there to listen, they were showing some respect for that.
You have to let yourself go...completely...and know that having even some talent and willpower is a LOT more than the majority of folks have who are there listening.
I mean...it's not like everyone in the audience is a master musician, and they're going to critique the shit out of you if you are not 100% perfect.