I'm nearly immune to stage fright because, even before a performance, I'm so focused on the music that I hardly notice an audience. Perhaps you need to learn to be single-minded about the music and consider the listeners to be irrelevant. While this is unlikely to be helpful with an impending performance, it can develop over a little time.
As your anxiety is beyond your control, seek professional help. Counseling, likely combined with medication, may be very helpful.