Hello friends. I've not been here for a while, and I can see that many of the most knowledgeable and helpful contributors have left this place quite some time ago.

Anyway, I bring a very specific question about mental practice and memorizing away from the piano.
I want to learn Chopin's Prelude in E minor, op. 28 no. 4. Now, this is a piece of ridiculous (technical) simplicity. I can sight read it perfectly, but I want to try to learn it completely away from the piano first. I've just started working on this approach after not playing piano seriously for a long time.
I can recognize patterns/memorize the sound of the piece quite well. However, I have difficulty memorizing certain sections. For example, the 8th measure has a very wide (somewhat random) melody, and other than a brute force "memorize each and every single note" approach, I do not know a more efficient way to go about doing this.
Likewise, with the chords in the left hand. These chords do not make sense to me. I don't know much harmony at all, and this is a harmonically challenging piece (jazz chords, anyone?) as it is, so I'm having a brutal time memorizing the left hand accompaniment. I've basically broken it down to memorizing which fingers move when the chord changes, without much meaning attached to them. I can identify the chords by name, but I don't really know why they're moving the way they are. Beating this into my head is taking a long time, and while at the end of the day I have it, it's not as secure as I'd like it to be.
In comparison, I'm learning Bach's C major invention using Bernhard's outline, and I have no problem with the memory. I can play it at light speed in my head, I can call out the motifs/harmonic changes as I play, I can play it slowly, quickly, blind, and deaf, and it is solid. I have a hundred and one different associations. I can write out the whole score, write it backwards, play each hand separately and sing the other hand. The mental practice approach has worked quite well for this piece.
The Chopin, on the other hand, is not going so well.
So, does anyone have any advice on how to learn seemingly random melodic lines and harmonic progressions one doesn't understand? The benefits of mental practice are truly incredible, but it is exhausting (in a different sense) and difficult to do if you don't know how to go about doing it. Nonetheless, I will persevere!