More seriously, ADHD, among others, is a working memory disorder. A way to cover this is to use "prosthetics" for it, such as a notepad that you always carry with you. If this affects you strongly, train your muscle memory to use it to note down what you were planning to do and to refer to it when you forgot.
In PC-speak, ADHDers have traded RAM for more CPU.
That's working memory
Or is it maybe that we simply rotate far too much stuff through the working memory? Where a "normal" person puts in two/three things and keeps a fourth on the stack, we switch our focus so often to different things, that we have to put a lot more in and out of our working memory, thereby also pushing out everything we meant to think of "later".