This advice from is just for you, bzzzzzt....
This is a natural progression we all have. Nothing wrong with it. It will pass if you keep working on it diligently and take a look at the broad perspective of progress in terms of months not day to day.
Also, sometimes sleep on it. Not meaning just over night. But sometimes working on something else for a week or two can, in some ways, make coming back to the Bach reveal that your nerves just needed to assimilate everything and get it set into you to actually deliver something you would appreciate.
Also, most importantly, what I learned through my teacher (from Juilliard, why I mention this? just because) told us in class that it is important that If you have these types of practices, is not to just glaze over it or do it half-baked bc you are just not getting it and frustrating you. Which is sometimes the intuitive direction to go. But to finish off that practice whether it is another fifteen minutes or another hour, strong, whether you broke through or not, or think you have learned something or not, bc, it is within that frustrating process you are engaging your brain in ways that it is beneficially taxing. Just like the muscle growth usually is due to that last rep that you cannot hold up but struggling to, not the 11 reps prior to it.