You shouldn't bite too much over your level. When I was two years in, I struggled on the C Major prelude at the beginning of the WTK...
But if you struggle with Rachmaninoff's Op. 3 No. 2...Do you have a teacher?
Well, then, I'm a bit confused about your level. So the only advice I can give is don't do something too difficult for your own good, find something in the middle, and stick with that because you don't want to overwork yourself. This could be the Chopin Ballade, the Polonaises I mentioned earlier, or even something easier. Just find what works - something that challenges you, but that you can get down eventually.
I don’t think you should learn the Fantasie or Ballade......learn to appreciate a good ol’ WTK prelude.