Same here, I thought it was great, especially considering that you've not been playing very long. FI is a above Gr8 level, and not many people achieve that level in 3 years. It took my sister more like 10, but then again she started at a very early age. If I can play FI when Iv been playing for 3 years, I'll be a very happy man!
But with respect to your problem, I can relate to this totally. Iv been playing just over a year, and are learning the Bach Inventions and easier Chopin Preludes (self taught also). I find that I can memorize them quite quickly and get them under my fingers without too much trouble. However, its been taking several months to get them up to a playable standard.
I'll tell what I think it is...
Playing piano seems to be all about solving problems. Initially the problems are obvious - learning the notes, developing the proper technique, considering the interpretation etc. However, when it comes to ironing out the little flaws, I think it must take much more experience as the problems are far more difficult to identify. I mean, saw you have a certain passage that you just cant seem to get right - as a beginner its just a matter of trial and error in finding out where the faults lie, right?
This is where I believe a teacher comes into it. They can direct you in identifying and correcting problem areas without need for endless experimentation.
Then again, maybe this is a double edged sword. It could be argued that taking this time in the early stages to develop your problem solving ability might serve well in the future. It kind of like learning to drive - when you have an instructor sat next to you the whole time, you dont really think for yourself. Its only when you get out on the open road for the first time (alone) do you REALLY start learning!
So I guess what Im saying is, just give it the time and work it takes to finish the job. It might seem to be taking ages, but dont forget, the pros can take years to get a piece upto performance standard.