Additionally, you can only read organ music quite slowly, perhaps too slowly for the best results, especially those with multiple sharps and flats. This is true not only for real organ compositions but also in playing hymns.
On top of that, lack of music theory foundation is causing you to struggle. This is evident from the fact that when you play from the score, it says very little to you. You can't recognize the patterns, cadences, musical ideas, figures, modulations, and colorful chord progressions.
And of course, you know what you should do - put in more effort and work harder but so far your progress has been too slow.
If concentration, theoretical knowledge, speed of reading music, and memorization are the things which hold you back from improving your success rate, then here are a few things I recommend to you for starters:
1. Learn the pieces in fragments of about 4 measures
2. Learn these fragments in solo parts, in two-part and three-part combinations first
3. Practice in a tempo slower than the concert speed by at least 50 percent
4. Each step may require 10 correct repetitions in a row
5. Practice sight-reading daily for at least 5-15 minutes
6. Sight-read pieces in separate parts first
7. Try not to look at the fingers and feet
8. Memorize the system of the circle of fifths
9. Learn a few chords and try to discover them in your pieces
10. Transpose one or two chords in as many keys as you can
Apply the above tips in your organ practice and with time you will be able to improve your success rate.
[HT to John]
Next: Dreams and sacrifices
4. Intermezzo (p. 19) from 12 Pièces, Op.16 by Leon Boellmann (1862-1897), French Romantic composer and organist.
Wake, Awake, for Night Is Flying