In music, most things are easy or impossible. If you are ready for it, it's easy. If you dont have the technique or theory, then it's impossible.
You have the motivation to work it out for 2 pages, so maybe it's difficult for you, but not impossible. 4 against 3 is a weird thing to get used to, and there are few other pieces you can do to prepare for that rhythm.
Therefore, I say keep at it! When you come out of it, you might not be able to play it from start to finish without stopping, or at the indicated speed, and you may have to fight like a tiger everyday to keep it in your memory. but it's impossible not to learn something while working on such a piece. I say learn it for your own curiosity, walk away, pick up something easier (that will seem even easier after playing FI), and then come back to fantasy impromptu. Then you will be able to play it and keep it. I hope you realize, as most people agree, that keeping any piece of Chopin for any length of time is very difficult, because it is so hard to memorize.
I also second Justliam's question - Can you play the whole mmonlight Sonata? I'm thinking that you can't, based on what your aunt said..
donjuan