It's like telling a fish to climb a waterfall. All saying "just do it and move on" does is reinforce the idea that the other person isn't good enough.

Try to do homework assignments at 6 PM get distracted realize it's 11 PM and time to sleep stay up anyway to do it get distracted every 5 minutes get back on track each time because you're incredibly stressed rake a nap because you're sleepy and can't think wake up at 6 AM realizing it's the next day and you only finished half the assignment be late for the bus by 1 minute shamefully wake up parents to drive you or walk to school end up being 10 minutes late for class turn in your half-complete assignment, which you'll probably get at most a 50 on sit through a bunch of uninteresting classes rarely be able to pay attention barely understand the material because of sleepiness go home repeat

That is only a slight exaggeration of undiagnosed ADHD. The most unrealistic parts are the length of the first distraction (should be 2 hours max) and the grades (probably around 70%). None of this comes from a lack of trying.

Here's an analogy of how distractions happen. Say that you're at home doing the dishes. 5 minutes later, you teleport about a mile away in a random direction. From then on, you teleport a mile in a random direction every 15 or so seconds. You eventually are teleported back to your house, where you can finally walk back to the kitchen and finish the dishes without teleporting for several minutes.

Usually, we think about a tangentially related topic over and over again until we're so far off that we realize what we're supposed to be doing. This is also one aspect of ADHD - it impacts so many miniscule things that add up to become giant problems.

If you don't think ADHD is a serious concern, you should look up some statistics regarding it. ADHD people have significantly lower life expectancies on average. More seriously, they are 5x more likely to attempt suicide than people without it. Also, people with ADHD are really likely to have other conditions/disorders as well.