I went through a good, long stretch where I knew more about poker but my overall play wasn't as good as it had been when I knew less... Whether or not they realize it, I think this happens to a lot of players on their journey from relative beginner to middling. As the newness and initial excitement of the game wear off, we get a little jaded and lose a certain edge that seriously affects our game as a whole. The difference?
As I (in my estimation) find the keys to push through the upper borders of middling and into the journeyman range, I find the ultimate key is long-term patience, even temper, focus, and walking away when I feel myself losing grip. I know it sounds obvious when it's spelled out, but I don't think most players in this situation even realize how often their mind is drifting or they're loosening up due to impatience. It takes a lot of focus to realize when you're losing focus.
I now play a lot of poker on Facebook, which I would NEVER have done before... Every time I start to slide into the bored, "doesn't matter" mindset I kick my own butt, refocus, and force myself to take it seriously no matter how donkish the play around me. It's been quite effective discipline training.