Some people say that it's not what you believe that defines you, but what you do. I like to think that we are best defined, not by the things that we do, but by all the crap we've been meaning to do.
(Note the change in phrasing. When you've actually done it, it's a thing. When it's yet another item on the backlog pile, it becomes "crap." That change isn't my invention.)
I struggle with this problem constantly. I think I waiver between a state of complete mental focus and terminal distractability, and the end result is that I generally can't hold on to a "to-do" item for more than a minute. If I remember something as I'm parking my car at work, I'll have forgotten it by the time I walk to my office.
And forget about leaving the house with everything I need.
I should really get a notebook and carry it with me all the time, so I can write all the things I need to do in it.
Note to self: get a notebook.*
*No, not a laptop. Actual pieces of paper bound together.