Wow, that sounds very familiar. I find that when I try to dive into long texts, even (especially?) those that I find very interesting and which I'm intent on finishing, my mind wanders off onto wild tangents within a few paragraphs. When I find something interesting that sparks another thought, I'll trail off on that thought instead of filing it away for later. I'll continue to read the words without even realizing that I'm not absorbing any of the content, which means that I'll sometimes need to re-read a passage three or four times. It's incredibly frustrating. You're right that skimming is surprisingly easier, though. I find that if I try to blaze through a passage and read only some of the words, my recall is often
better than if I'd tried to read at my normal pace. Less chance to focus on something else.
I used to love reading books as a kid, but I started with the above somewhere around high school. I think that the steady stream of novel (and thus exciting) information available on the internet has had an detrimental impact, but of course that can't be all of it. Still, I've lately been trying to limit the quantity of information that I attempt to take in by, say, opening only one or two news articles or HN links (instead of every one that looks interesting), and I've felt more clear-headed since.
May I ask when you put a name to the problem and started with medication?