Stress. You do get some things done, you say. but stress will interfere with the thought process and ...well for me, it can even make me "blind" to seeing something physically right in front of me on the desk! Maybe because you do categorize some things as "simple" that adds the "of course you can do this" demand upon yourself... and the stress blocks it.

Breathe.
Give yourself time.
Be patient with yourself.
Remember, it isn't you, it's the disorder.