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 go through phases of writing things down. I do it for a week, realize I remember everything I wrote down, and get used to remembering. Then I'll think, well, this whole thing was useless, I'm just remembering it. So I get lazy, stop writing things down, and start forgetting. Repeat ad nauseum.
I do something similar for waking up on time. I'll set 8 alarms in 15 minute increments. I'll start waking up 2-3 minutes before each one, and think to myself, well this is annoying, I'm waking up on my own. So I remove most of the alarms, and then oversleep.