I've been working on these two bars for a few weeks and I can now say I can play them. I suppose only someone who has tried and succeeded at this will understand my sense of accomplishment. I learned the right hand first. Then I figured out the left hand. Then I put them together, which was really, really hard. At first I thought I'd never figure it out. But I remember learning another (much easier) piece about a year ago, which I can still play today, and I had the same feeling: that it was impossibly complex and I could never get my hands to operate independently enough to pull it off. But each time I tried, I made a little bit of progress, enough to keep me at it.
Coincidentally, my daughter was working on her axel this week. (She's a figure skater. An axel is a jump with one and a half rotations.) On Monday she landed it twice. Then on Tuesday and Wednesday she couldn't do it. It was very frustrating for her. Thursday is her day off. Friday she tried and failed again. Then on Saturday she nailed it. Several times. And yesterday she was still landing it. Now, she doesn't always do it equally well, with equal grace. If it had been in competition they'd each get a different amount of points. But she is feeling the satisfaction of crossing the threshold between not being able to do it and being able to do it. It's a joy to watch.
I feel like I will ruin this post by explaining what this has to do with writing. The important thing is that you sit down at the machine every day and try to perform something. With that two-part invention, there are forty-four notes altogether, and the trick is to play them all in the right order at the right time, often two at a time (one with each hand). So you have to have a strategy for which fingers to use when, making sure they are available to play the notes comfortably as you need them. (On my sheet music—not the one pictured here—there are numbers that suggest what fingers to use. That's a big help.) Here's how Wikipedia describes the axel:
To perform an Axel, the skater typically approaches the jump on a right back outside edge in a strongly held check position before stepping on a left forward outside edge. He or she vaults over the toe pick of the left skate and "steps up" into the jump with the right leg. The skater crosses the left foot in front of the right, which is known as a back spin position (similar to the loop jump), to bring the center of rotation around the right side of the body. This act is often described as a weight shift in the air. Uncrossing the legs during the landing checks the rotation and allows the skater to flow out of the jump with good speed.
So when you're practicing for either an axel or the first two bars of Invention No. 13, it's clear what you have to do. You sit yourself down at the keyboard, or you lace up your skates and get on the ice, and then you have a certain amount of time on a given day to do it. And for a while, many days in a row, you fail. (During that time you sometimes forget that until you started trying you weren't failing at it, you were succeeding at other things.) And then, suddenly, you don't fail anymore, but rather do it more or less well.
From then on, it's one of the things you are able to do. You're not always at the top of the game, but you have this move as part of your repertoire. (Of course, you have to keep practicing or you'll lose it. You can't expect to play a piece of music after a year away from the piano. You can't expect to land your axel on the first day back after a year off the ice.) This is all common sense stuff. It makes intuitive sense.
Just try to think of your writing that way too. Think of the problem of writing a particular kind of paragraph (about the background of your study, or your theory, or your method, or your results, etc.) like the problem of playing a few bars on the piano or completing a figure-skating jump. Try to make the task as straightforward as the information provided by the sheet music, or that description of the jump in Wikipedia. Have a clear idea in mind when you try to write. At one level, I could "read" the music. But I did not know how to play the notes. That's like having an idea but not being able to put it into words.
So you have to sit down and try. Again and again and again. And when you're satisfied ... well, that's a really satisfying thing.