You mention never feeling like you've mastered a piece.  My teacher has told me many times, with etudes and with the pieces I work on, that we will come back to them more than once.  I also feel that I would like to have "mastered" a piece before moving on, but I now realize that will never happen.  I'm trying to rethink this goal from mastering to working a piece until I'm happy playing it.  As I've progressed a little I understand the wisdom in what she says (not that I've ever doubted her).  But at times if we've worked on a piece for a long time, I can now understand that at this point I've given it all I've got and the only way it will get better is when I've gained new and better skills.  This is very freeing mentally because I know it's not that I'm always going to play it poorly, but that I don't yet have the skills necessary to play it well.

I hope this helps.  Enjoy your journey.