brain shuts down with engine

I'm having an embarrassing problem. My brain seems to shut down lately
as soon as the key is out of the ignition. It's little things, like
leaving my kneeboard (with gas card) or charts in the airplane,
forgetting to completely tie down the aircraft, forgetting to properly
fill out club paperwork, etc.

It's all getting embarrassing, and I'm starting to get a little
nervous. So far, I have not done anything stupid in the air -- that I
know of -- but, I do wonder if I will.

A possible factor is that I'm flying much less than I used to, always
alone, and usually with very tight time constraints.

Any advice? Does this happen to anybody else? How do you stay focused
until your outside the gate?