ADHD

Not saying that it is the case here, but It is entirely possible for people to sympathize with your condition but still not being able to understand it because they have no point of reference. An example is the phrase "crippling pain." Most people will think only of the somatic meaning of crippling, and think that because someone can still walk normally they're not experiencing it. However, there's also the psychological component: pain so persistent or insidious that it stops you from thinking straight. It affects your performance when you have a white collar job, it affects your mood, and it carries a lot of stigma and judgment because it's not a visible disability. Heck, younger me was guilty of such preconceptions too. It didn't mean that I didn't care, but I had no way to mentally frame it within my own experiences. I still don't, exactly, but I'm able to extrapolate the scale from other experiences. In other words, I kind of understand now to a degree that I can avoid sounding condescending when trying to be sympathetic.

Again, I'm not saying that it applies in your case - I only have a couple of paragraphs from your post to go by. I'm only trying to convey that there is a difference between understanding and sympathy, and in many cases it takes education for someone who doesn't experience ADHD to understand it. Sometimes it's a futile exercise, yes, but in the cases where you can get through you'll form a much stronger bond.