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 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