It sounds like you're having trouble conjuring up images based on plain text. I don't know, but could be some kind of learning disorder thing, or your brain is just a bit "different". I don't mean this to sound offensive, just an honest guess. If it's really that much trouble where you can't even enjoy reading, perhaps talk to a teacher or tutor or some kind of professional.

With me, I can glance at a whole paragraph and kind of absorb separate fragments of the sentance at once. But I was already reading books without adult guidance at the age of four. Simple kids books, of course, I'm not some kind of genius. Just been an avid reader for 30+ years now, and happen to have a very strong imagination visually, which helps.