It sounds that you are trying to cram the piece into your head as semantic knowledge, that is to say a string of facts ("...then comes the G7 cord...").
I do not think most -- or possibly any -- accomplished musicians store whole pieces in semantic memory. They store it in procedural memory, and they store it, above all, as the series of sounds of the piece being played!
Off the top of my head, one thing you can do -- which I suspect you haven't done if you're leaning so hard on semantic memory for memorization -- is to get a recording of the piece you're trying to memorize, and then listen to it all the time. Put it on endless loop on your commute. And mentally play along as you do -- or sometimes physically play along, either at a keyboard, or on a table silently. Sing it to yourself in the shower. In doing this, you're engraving the sound of the piece into your memory, such that you can remember the piece by listening to it from memory. And that's what you should be playing from.
If you can't find a commercial recording, you can record yourself playing it; even yourself playing slowly, or poorly, can be better than no reference recording at all.
And I'd like to add, this is the sort of memorization which pays dividends in expressiveness and interpretation. Music is not a string of facts, even if that's how midi works. Music is sounds, and ultimately you need to experience the music as sounds and engage with it directly that way. To make music you have to free yourself from the score (or at least, it's vastly easier for most people to do so) and from the representation of the sounds as marks on paper or letter names or solfege or any other mere conceptual allusion to the actual sounds which comprise the piece. These things, phrases like "G7 chord of three notes", they are names for these sounds, but not the sounds themselves, they are the signs but not the signifieds, they are pointers, not the values. You must engage directly with the values -- the sounds -- themselves, to inhabit them and bring them to life.
And probably to remember them at all in the first place.