Unfortunately, our brains have not kept pace with "modernity".<g>
If we are prone to "nerves" faced with performance before an audience, the primitive part of our brain tries to protect us by putting us in a "fight or flight" mode, i.e. prepare to fight the predator for your life or run like hell.
Presumably, in this situation, the brain percieves the audience as the "predator" and bio-chemically gives you the choice of "slaying" your audience or "evading" them.
But intellectually, you know you must stay and play, so what to do with all of that unnecessary, life threatening, mind/body preparedness?
With cortisol and adrenallin and I-don't-know-what-else directly injected into your blood stream by your brain intent on "saving" you, all it does is make you miserable, and lacking a good deal of the presence of mind you have in your normal practice situation.
Then, of course, as mistakes are made and/or the piano seems like another predator, this compounds the problem.
Thank you, Brain!So the only thing I know to do is to acquire experience before an audience.
Also, you can practice this at home by purposely "making" yourself nervous as if you're in front of an audience, and learn to perform under this type of duress.
Try and play slower, and make sure the pieces performed are WELL within your grasp.
And the sooner you can "immerse" yourself in the music the better as you "direct" yourself conciously until you can allow "auto pilot" to once again run the show.