Accepting the framing that ADHD does exist, it feels like brain damage. Every year growing up, in any group of kids I was among the most immature, the most off-task, the most distracted, and the most forgetful. I am plenty smart, move very quickly, and have accomplished plenty, but my entire life has been defined by being more bored, forgetful, and immature than nearly every single person I've been close to. My wife (performs very well on relevant metrics) says it's extremely noticeable and unusual from the outside. Over time I've learned to heavily lean on coping mechanisms (my phone rings about 35 times per day with timers I snooze tactically, I leave my keys in my bag any time I go places so I can't leave without the bag, etc.) No amount of character development schemes, punishments, shaming, positive skill practicing etc. has changed my natural proclivities in the slightest, although many days of good sleep and diet can sometimes produce a bit of a sustained flow state. Sometimes I wonder if I just lack some kind of ion in my blood.
My coworkers sometimes tell me they feel like they have ADHD. Then I watch them listen to someone talk about a subject they find boring for 5 minutes without drifting off or fidgeting, and then remember off the top of their heads to respond to outstanding emails. Come on guys, if we're going to make up conditions we should at least reserve the label for people that it describes accurately.