Well, I know how you feel. And I am diabetic. Not saying with any authority that it may be your problem, just someone to compare yourself to. I cannot concentrate worth a damn, honestly moving the mouse from one icon to another gives me enough time to lose concentration. Short term memory while not awful, just doesn't exist unless I get it in mind that what I need in memory is important, although I thinkk that is more of a concentration problem.
Honestly, I've felt worse since summer, when I am on a pretty regular sleep cycle with enough sleep per night (11:30 or so until 7). When I was all over in school, I felt much better.
Just another thought, how physically active are you? I've slowed down a lot over the summer, and I do think it is part of my problem. Winter time, I was skiing or playing hockey or snowshoeing just about every day. Now, I work all day, come home and sit all day, and feel like crap.