There will always be pieces, that you cannot learn in one go. I do not know, what level you are, but this could possibly apply to all levels.I have several pieces "in the oven" as I call it. Those are pieces that you , for some reason, cannot get further with. As soon as you feel you are getting nowhere, and you feel you have practised as well as possible, give them a long rest. In my case there were for instance the Chopin Preludes, the whole set. Some remained elusive for a long time, but after a couple of tries, usually half a year or so, apart, they eventually came right. Of course you just aim for those "bad bits". Some people may take 10 years for particularly demanding pieces, like Pictures at an Exhibition-Mussorgsky. Not for the whole work, but for the bad bits. One keeps returning to it at long intervals. I think, when it is really too difficult for you, you will not be able to make progress on it in that way either. You could also try to find something that has a similar problem in it.
Or create exercises that may overcome the problem. Analysing and understanding what the problem is, is essential.