The dream that Margaret has is common to many organists. But it's not so easy to make it a reality. So often people can only play rather slow music and when they try to play faster, lots of mistakes appear. This is frustrating.
If you experience such challenges as Margaret, you have to understand that it's better to play slower than with many mistakes. Therefore, choose the tempo according to your level of ability.
By repeatedly practicing very slowly and reducing the texture to single voice and various voice combinations, you will be able to eliminate mistakes and reach the level when you can play rather slowly but fluently.
If you want to play faster, perhaps you need a) to work on your technique and b) to practice your pieces at the concert tempo but stopping and waiting at the smallest fragment imaginable - a quarter note.
Once you can play this way until the end of the piece at least 3 times without mistakes, stop every two beats, then one measure, two measures and so on always expanding your fragments and playing the music inside the fragment at the concert tempo but stopping, waiting and preparing for the next fragment.
If you want to memorize music easier, you have to develop a systematic procedure of practicing short fragments 5 times while looking at the score and 5 times from memory. Usually the longest fragment you can remember this way is one measure. As you might have already guessed, after memorizing one measure fragments, start expanding them little by little from memory.
If you haven't done so, try to learn something about keys, chords, chord progressions, cadences, and modulations. This will help you understand how your piece is put together and consequently facilitate the process of memorization.
3. Begib mich nit myn höchster hort (p. 26) from Buxheimer Orgelbuch (ca. 1450), a German Renaissance collection of organ music.
Hark, the Voice of Jesus Calling