ADHD is a difficult topic to discuss on HN. I'll preface this by saying that I'm not doubting your situation, or any other commenter's particular situation. This [section of my] comment is meant to be general:
In the context of this mentoring group, we go through phases where almost everyone suspects they have ADHD for various reasons. This is usually triggered by one of two things: Either someone shares an online "Do you have ADHD quiz?" that is sponsored by Takeda or another ADHD medication manufacturer, or a front-page Reddit infographic misrepresents ADHD as something like "Do you some times forget people's names? Maybe you have ADHD!"
The reality is that ADHD is very challenging for those that have it, but the pop-culture definition of ADHD has become so vague that people who don't have ADHD are increasingly convinced that common life experiences are symptoms of ADHD.
Focusing is hard. Studying is hard. The Grind is hard. It's normal to struggle to focus, but it's even more of a struggle for those with ADHD. However, having to work to focus for extended periods of time, in and of itself, is not an ADHD symptom, it's just life. ADHD is a much more severe impediment.
(Again, not referring to the parent comment): Anyone curious should avoid self-diagnosis and seek a trusted professional. Ideally not a family doctor who simply writes prescriptions on request, but someone who can recommend self-guided therapy programs and combination treatment. Adderall isn't all it's made out to be, especially after the initial motivating effects wear off and you're left with the realities of long-term stimulant use, which are nowhere near as exciting as the first few doses.
> What if they are, though? What if it's like height, where genetics sets a range and environment picks a point in that range? If I had starved as a child, I would be shorter than I am now. But if I had eaten more, I would not be taller. I'm about the same height as my parents are.
Genetics and upbringing may set a baseline for focus and motivation, but those traits are demonstrably not set in stone. Contrary to your example, diet does have a significant influence on height, but it's not the sole determinant.
Height isn't a good example, though. Consider something like running capacity. Some people are naturally more athletic than others, but barring severe disorders, everyone can develop more running capacity through training. Someone who gives up and never tries to increase their capacity may not believe this, but it's true. An average person can't simply work their way up to competing with Olympic sprinters blessed with perfect genetics, but they can significantly increase their running capacity from baseline by putting in the work.
Likewise, attention is a learned skill. Some have more baseline attention span than others, but it can be increased through training and practice. ADHD modulates this, but it doesn't prevent practice from helping. If anything, people with ADHD need to invest more effort into training their attention spans than those without.
> Willpower doesn't seem to come naturally to me, even after 10 years as a professional programmer. Adderall didn't help
Adderall and other stimulants don't provide willpower, contrary to popular belief. Only people without tolerance will experience a temporary motivation boost from stimulants. This effect diminishes as tolerance sets in, which is one of several reasons why drugs like Adderall aren't successful for treating disorders like depression.
Willpower is another learned skill. Expecting it to come naturally won't work forever. You have to learn to embrace the grind, do the work, and power through the urges to give up and do something easier if you want to get anywhere.
> I don't exercise at all because it's not my interest.
The reality is that the things we need to do aren't always going to line up with the things we like to do. You're lucky that you have a natural interest in programming, but you can't expect every necessary activity to have a natural interest behind it. Some amount of physical activity is essentially required for a healthy existence. You may not be interested in it, but that doesn't exempt you from requiring it and it certainly doesn't mean you won't benefit from it.
Some times the things we have to do in life aren't immediately enjoyable. It's on us to find ways to make them more enjoyable (e.g. find a sport you like, or take up walking), and some times we just have to do the unenjoyable thing for the sake of progress.