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.
I find that the act of writing it down makes it more likely I remember it. And since I'm always writing everything down, I always know that I can reference anything. So there's no "remembering I wrote it down," it's always going to be there.