I've had this approach to books for a long time: if I crack one open, I have to get to the end, comprehension be damned. This is completionism. And I ascribe it to an attitude I picked up somewhere along the way that I want to have read a book rather than to truly read a book. 

Part of completionism as a mindset is the idea that you can collect knowledge like little ornaments that you show off to people. It encourages you to fill up the bookshelf with impressive works so folks can see how smart you are when they come over for dinner. I am guilty of this especially with writers like Lacan, Heidegger, Deleuze, and others.

A haunting question: How much do you actually retain from the words that you read? The purpose of reading and learning is supposed to be knowledge assimilation, not moving your eyes over words on the page. If that means hopping around books and leaving some incomplete because you either lose interest or you've understood the idea and you don't require further elucidation, then fine.  If it takes a year to get through a book, that is also fine. 

I'm trying to reshape my entire approach to learning to be more about spending the time with concepts to ensure that I actually understand them, rather than "getting through" a number of pages or a number of books. One of the better things I've done for my fitness was I reshaping my goals to be about training duration rather than miles or number of push-ups or metrics like that. I'm doing the same thing now with reading. 30-60 minutes every morning, even if all of that is spent on a single page.

Encoding knowledge into your long-term memory is about marination, repetition and struggle. That just takes time.