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 not what it feels like for me. I grew up basically getting the "what's wrong with you? Just focus" treatment, so I always default to saying to myself "no, no, it'll be okay, I'll just try really super duper hard; I'm sure it'll work this time."
Yeah exactly the same experience for me. It's not about forgetting it, it's about thinking that if you just tried a little harder you would be able to do it. Because that's what everyone kept telling me. And even now it feels like i'm using my ADHD as an excuse to be lazy.