What can you do to help yourself in such a situation?
Well, ask yourself, how are your harmony skills? It's important to assess them because they are related to playing in public.
People often learn the music note by note, perhaps listening to the beautiful sounds that the composer notated. However, it's very rare that a person would pay attention to the harmonic outline behind the notes. It's very rare that an organist would try to follow the functions of chords, chord progressions, cadences, modulations and other concepts of harmony.
Because let's face it, this kind of approach requires specialized knowledge which many people don't have yet.
Usually one would just learn the notes and try to recreate them in private or in public in the best way one can.
That's a good start but not sufficient for a calm and convincing performance.
If you study harmony at the same time as organ playing, then it helps you understand the music that you play.
One more thing really helps here - transposition. Try to transpose the piece into as many keys as you possibly can starting with zero accidentals.
Then see what happens.
See what happens over time to your performance, to your technique, to your mental understanding of the music.
I predict that chances of losing your train of thought and playing note for note during the performance will be smaller because you will know the meaning behind the notes.
[Thanks to Alan]
1) creating a vision, goals & plan for yourself as an organist at a level appropriate for you
2) how to practice efficiently
3) sight reading, how to do it, how to improve, how your eyes scan/move across music (esp with open staves)
4) hymn playing, basics of how to prepare and play, with and without pedals, how to play hymns at short notice
5) public concert performance, dealing with nerves, how to connect with audience, playing strange/new organs, preparation of program, how to deal with a crisis, how to eliminate mistakes.
6) finger and pedal technique, exercises to improve technique without spending hours playing Hannon.
7) improvisation, how to get started, how to plan one, how to practice art of improvisation in private, how to create an improvisation from a simple hymn tune or catchy melody
Thanks to John Higgins who submitted these thoughtful questions. Join us if they seem relevant to you.