These 2 statements alone display your impetuosity and lack of humility and patience and your part. I was once in your shoes, impatient and overestimating my own abilities, I picked overly difficult pieces to learn thinking that they were 'easy'. It's easy to deceive one self into thinking that one played well without feedback from others, I knew because last time I used to think I played 'well' until I started listening more to other pianists play and some well-meaning teachers gave sound advises to me. I have since matured in my thinking and for one thing, making good music is my main goal, not trying to play harder pieces.
My main advice to you, ask yourself are you serving your own ego or the music? I know its emotionally rewarding to play technically difficult pieces as I experience before, but I knew I was cheating myself, I was depriving myself of good and balanced musical development by my stubborn attitude, which seem to be in common with you now. It takes a lot of humility to play piano well, ask yourself, say if you get the best teacher on the planet and he gave you say a very simple mozart sonata to play, will you be insulted because it's too 'easy' for you? Or will you be joyous that you are able to learn new musical insights in the sonata you are going to play? Enough ranting, ponder what I said just now.

I'm impetuous, and lacking in both humilty and patience... wow... cool.
1. impetuous can just mean characterized by sudden forcefulness of emotion (i.e. passion) so i'll take that one
2. Personally, I would assume a massive declaration of personal character based on less than a paragraph of information cut out of a forum post to be a sign of a lack of patience and humility... but thats just me. I don't overestimate my abilites pianistically. I know what sounds good as I spend time reading and listening to both classical music literature and music. I get feedback from plenty of people.
3. It is almost always a mistake in stating a point to assume that your situation mirrors someone elses closely. It almost always differs to some degree.
4. It matters very little wether I am serving my ego, the music, or the man in the moon. All the preceeding are statements of value and in this case they are based on subjects without absolutes. In other words, as far as music is concerned; to each his own.
5. It doesn't take a great deal of humility or anything to play piano well. (there are only 3 types of pianists..., godowsky's passacaglia is impossible... etudes impossible... - Horowitz)
6. I play what I want to play for reasons I find acceptable. This isn't systematic theology. There are no set rules which govern music or how it is to be played. (if you think these rules exist, check out music from other regions of the world and see how they offend your sense of what is and is not music.)
7. Chang's book say that the Fantasie really isn't that hard, so im not all by myself here.
8. I'm not going to piddle with a Mozart sonata... I'd rather get to the edge and be playing Godowsky's passacaglia in a few years rather then be worried about the easier stuff now. I always have time to go back and pick up the easy stuff.

9. I need a cookie... someone please make this post end and then get me a cookie.