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.
How does writing things down help when I don't remember to read them back...?
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.
The idea is that you can regularly check the notebook. If that doesn't work for you, then an app with a notification might be a better match
I started using todoist for this and it helped a lot.
(I'm not trying to shill this product, but I am trying to shill the idea - I'm sure there are a ton of similar products out there.)
I use it in the "getting things done" method where you drop every little thing you need to remember in the inbox and go through the inbox once a day to organize your thoughts into whatever project or container they need to be in. It has immensely improved my life and the underlying feeling of dread that you missed or forgot to do something.
I don't think I have adhd, but I do have pretty severe memory problems. I've managed to get into the habit of checking my notebook every time I think about and/or touch it. I always keep my book on my person, or with my phone so I don't lose it