You almost have to learn to read again. I regrettably stopped reading for years (other than web articles, email, skimming, etc). In the last few years I started reading novels for pleasure again, and I swear if I stop for a couple of weeks when I get back to it I have to take my time and be slow. Something that has helped is reading books in my native language (Spanish). Since I'm not as strong as I used to be in Spanish, I find that reading in Spanish really forces me to take my time reading every single word. Resist the urge of your brain / eye saccades to skip ahead a bit. Keep reading and soon you'll realize you can do this for many pages.

Edit: I saw that you read plenty for pleasure, so it's probably not this :/