You probably don't have ADHD!
Or more precisely, whether you would get a diagnosis of ADHD if you consulted the appropriate specialist is probably irrelevant to your life.
You are one of the many people who have repeatedly derived pleasure from visiting HN. It is natural for humans to get pleasure from learning new things and getting fresh perspectives from other humans.
And HN is easy and comfortable and very safe. There is nothing at risk: your visit to HN will not end in pain such as the pain of trying hard to achieve a goal important to you and failing. Yes, understanding the implications of something you learned here can require mental effort, and mental effort is painful (at least momentarily), but the mental effort is discretionary: if you choose not to make the effort, the consequences (namely, having a less accurate model of reality than you would have had if you had made the effort) are far away and minor.
Things that are pleasurable and require no effort (and no danger or risk) are bad for people. Specifically it screws up the system in the brain for motivation, drive and reward. Indulge enough in these effortless risk-less pleasures and your motivation becomes weakened such that even if you go cold turkey on the pleasure, it takes weeks and weeks for you to regain the natural human ability to remain motivated by things that require sustained effort, pain and patience.
Actually it is worse than that because pleasure causes reinforcement of the sequence of actions that led up to the pleasure. Suppose for example that you make a mental move in your mind along the lines of, "my situation is hopeless; I'm stuck, and there is no way out," while you are interacting with a web browser. Suppose further that your next move (while in this state of nihilism) is to type "news.yc" in the location bar, then 30 seconds or 60 seconds later you notice a story or a comment on HN that explains some aspect of reality that has been confusing you and nagging at you for years. Your discovery of the explanation is pleasurable (and should be pleasurable). So far so good (you learned something) but pleasure reinforces the mental moves you made that led up to the pleasure, including the nihilistic move. If that nihilistic move gets reinforced enough times in this way, it can take years to correct the problem.
The point is that most of society is probably dealing with this problem. It's not that you have a mental disorder. Well, I don't know you: maybe you do have a mental disorder (ADHD or otherwise) making your problem with HN worse than it would be if you did not have a disorder. But people with a mental disorder have the same mental architecture for motivation as the rest of us, so I have to believe that they should control and pay attention to their participation in effortless risk-less pleasures as strictly as the rest of us should.
Some pleasures are better than others: for example, it is better to get pleasure from learning things than from harming other people (and at least some of the pleasure people get from video games is the pleasure in harming simulated people). But even a good pleasure can be a problem when a person has essentially unlimited access to it; when it does not cost anything and carries essentially no risk; when there is no painful effort or patient waiting between the moment the person decides to pursue the pleasure and when the pleasure comes.