Heightened levels of adrenaline and feelings of anxiousness are common and normal for performers at any level. How to deal with performance anxiety is the subject of hundreds of articles and books. Here’s one more…
I think that rather than only working on ways to decrease arousal, one has to learn to sing WITH it. Then you have more options, knowing that you can sing relaxed, excited, nervous, etc. There are mental and physical aspects to these emotional states. The way emotions affect us physically can be replicated without actually working yourself into those emotions, although if you are a drama queen you can practice actually making the interfering emotion happen as well. I had a teacher who suggested running through audition repertoire after running up and down several flights of stairs, for example. I use that one, or something similar that gets my heart beating and breath moving fast. I do not use his idea of setting the alarm for 5:00 and performing right out of bed!
You can probably come up with some other ways to replicate the bodily state of adrenaline so you can play with it. Then when you are about to go onstage and you feel like there is no air and you’re about to poop your drawers and your heart is fluttering, you can say to yourself “well, it’s not ideal, but I know how to work with this”.
This is a process of not denying the fear. Sure, we would rather never feel anxious at all, but if it comes you have to deal with it. Sometimes when it is approaching show time, or I have to sing for someone new, I feel calm and confident. Then all of a sudden, the anxiety shows up and I think “Well, it’s about time!”. It’s a rare time that I don’t feel it. And I’ll bet I’ve also said to myself 200 times “Why do I keep doing this.” That silly, hilarious-in-hindsight dread. And then I begin, and I usually feel better. I ALWAYS program something easy to start out a recital, coaching, or audition. Success builds confidence.
It is also essential that one be as prepared as possible. Anxiety based on lack of preparation can indeed lead to a horrible experience. And even if you have no stage fright at all, a lack of preparation can lead to a horrible experience for the audience!