I've been playing now for more years than I care to remember (not that I've got that much better!), but it seems to me that once you master something you get kind of stale, like you've reached a plateau. Then something else catches your interest, and you start to learn that, and you climb to the next plateau. If all else fails, try a different style of music, like acoustic guitar or jazz or something - even if you don't like it, you'll learn something from it that you can incorporate into your own playing. Good luck!