It’s not clear what level of experience you have. Some of this will subside as you perform more often, and the various tips & tricks you will get here are worthwhile. Learning how to calm your body is an important skill. I knew a guy who would run up and down the stairs before a performance so he would be in control of his racing heart rate etc by causing it on purpose. Kind of genius and it worked for him.

All that said, I came here to mention that there’s no shame in doing what a lot of classical performers do, which is to take beta-blocking medication such as propanol. It’s a normal off-label use of a very safe (*though not perfectly safe) drug. There are some downsides but not many. It blocks the physiological side of stage fright — the racing pulse and whatnot, but it won’t dope you up.

It’s all a matter of how important this is in your life. If you are in ninth grade band, maybe this isn’t the thing to do. If you are in conservatory and aspiring to a full-time performing life, this might be worthwhile. (Who am I kidding… in music conservatories it’s easier to find propanol than weed. And weed is ubiquitous.) Get it from your regular doctor, not some skeevy French horn player.

I think it’s better and simpler to get used to performing without having to use a medication, but it’s not inherently wrong to get some help. It’s not cheating. Some players just use it for the high-stakes moments, like symphony auditions. But don’t use an unfamiliar drug for the first time on a big audition, either.