Is this common within adhd or is this my stupidity

I always mix up what others are saying. Or never fully understand it. Like for example, I was told to stack paper in numerical order. I get stuck on which way is smaller to bigger, so I just did it the way I thought. Which was wrong. I also always forget instructions so I’ve been told this multiple times but still never get it right. I’ll also get stuck on some details that cause me to like mix it up further. Like when someone means one way, I think of as the other way.

I don’t mean to seem rude, when I do this but I also always walk out in the middle of the conversation. I can’t tell when it ends or not, so just in the middle of it, I’ll just start walking away. I can’t tell if this is just my stupidity showing atp, or does it have anything to do with ADHD. Which I got diagnosed with, but I don’t even think I have it myself? Everyone I know who has it is very different from mine, and I’ve been told that I probably don’t have it by multiple friends who do. So it makes me feel like it’s me being dumb.