Difficulty concentrating: A mental slipperiness, like trying not to think of something, or clench water in your fist. The harder I attempt to concentrate, the more prone my mind is to go zipping off in another direction entirely. I tend to be most productive when I have a large number of potential tasks to work on, and one critical one - I'll get all my lesser tasks done in no time at all, as my mind careens off the critical task over and over again. I'm least productive when I have a single task to work on, particularly if it's trivial to do. Indeed, the faster I can get it done, the harder it is to do it.

Curiously, I have -zero- problems with difficulty concentrating when extremely tired, and if there's something I really need to do that I'm making no progress on, I'll generally do it around 3:30-4:30 AM.