You've got to lower your ambitions even more. The most important motivation engine is the feeling of making progress - no matter how small. Try to analyze your difficulties even deeper than before: is your problem really a whole bar? To me, it is often just one little movement, one transition, that makes the whole "problem". And so I end up playing just two or three notes in a sequence, over and over and over and over again. Ta-dah-dah, ta-dah-dah, ta-dah-dah. Quick and slow. Sometimes the keystrokes have to be doubled or tripled, or been played punctuated and in other versions, in order to keep your brain "active".
And when you feel you make something better than before, you must give this attention and give yourself a little mental reward. The usual problem is that we ignore little progress because we focus on bigger ones, and hence we get disencouraged and hence we lose our temper, get frustrated and BLOCK ourselves. Compare this with a dog that you train - if you demand too much from him at a time, it will be too difficult for him, he won't get the reward when he needs it, and so he will lose motivation and walk away. You are not different from the dog, you need to get encouraged as well.
... and if you find this slow, baby-step way of working just too frustrating because you get impatient, you must play pieces that are more easy. Personally I don't see the point in playing things I don't want to play, in order to learn how to learn things I really want to play, instead of working more with things I want to play ... but that's me. I can accept that it takes forever to learn some things ...