I had a similar drop off in high school. For me it happened when the volume of assigned reading, particularly summer reading, crowded out all of the reading for pleasure. I just never started back up after that, despite fits and starts over the years.
I decided that I wanted to change about three years ago.
The first thing that helped was finding a book that I really wanted to finish and that I saw as a challenge. I tend to be motivated by challenges. Instead of “I want to get back to regular reading”, it was “I want to finish this one book no matter what”.
Second, I bought an e-reader that let me set the margins and text size to old man settings, such that there are only a few lines per screen. My mind wanders easily, especially when it feels like there’s an ocean of text on a page, so less text and frequent page turns helped me keep my momentum up and avoid constant wandering and re-reading.
As much as we all hate commutes here, getting a job that has me back in the office has had a silver lining in that time on the train is scheduled, routine time where I can’t do something else. I made the decision that it would not be dead time, so I religiously put my phone in my bag, get my book out, and focus on it for the 40 minutes until my destination.
I am a relatively slow reader like you, but much like exercise, sticking to a routine has meant slow but noticeable improvement. I think of myself as a book reader again, and I’ve made it over the hump where sustained concentration feels effortful. You can do it!