I agree with the juuf. A couple of years ago, I was trying very hard (and mostly failing...mostly) to learn Chopin's Winter Wind Etude 25/11. In disgust, I abruptly quit playing it. Six months and four easier Chopin Etudes later, I began work again on the 25/11. I mastered it in a month.
Try this. Make sure you memorize the piece @ a tempo slow enough to let you play the correct notes; take it as slow as is needed to get accuracy.
Tempo does not mean anything at this point!!!!!
After a couple weeks of ultra-slow, ULTRA-ACCURATE practice, drop the piece completely for at least three weeks and for as long as a year.
When you return to the piece, treat it very carefully, like wet paint. You will need to do whatever is necessary to neatly cure this musical 'painting' into a firmly fixed, framed and finished product.
I know this is rather generic advice. If you have a specific problem, e-mail me.
p.s. Just remember, if after two years you haven't made significant weekly progress, you're doing something wrong. If you can figure that out, you're
more than halfway there.
Per Sapientiam Felicitas!