I got JLPT level 1 in 2006. This probably represents the high point of my achievement in Japanese. I’m just good enough now to intimidate the people who plateaued lower than me, but not nearly so good that people talking to me forget it’s my second language.
There’s a point in language learning which I will call the Salieri point at which you’re just good enough to realize how much better the really good people are. I suppose there’s a lesson in behavioral economics to be found there – because my reference point has suddenly jumped way above my head, I find myself much less motivated even though that jump has only come as a result of objective improvement in my Japanese comprehension. If there were a way to put this into formal terms and study it, I bet it’d be revealed to be a major cause of fossilization.