I taught myself how to play piano a few years ago. My biggest challenge was motivating myself to learn from sheet music. I just found learning from the sheets too time consuming, and just ended up learning by ear. Eventually that turned into making my own arrangements and after that it turned into improvisation. Now that I've been doing this for so long, when I try to learn more complicated pieces from sheets I'm finding it way more difficult than I used to. Having said that, learning piano has definitely been one of the best investments of my time. I'm not an excellent pianist, but people are always fairly impressed with what I play. However, to a classically trained pianist I play with a lot of bad habits. I don't find it a big deal though. It doesn't limit my playing at my level; like yourself my goal was to learn for myself, not with the intention of making a living.