Studying is a skill and you may have lost some competence with it over lapsed time. I recommend looking at studying as a way to cultivate your capacities for direct attention and comprehension. You wouldn’t expect yourself to be able to sit and meditate skillfully for an hour a day, say, if you hadn’t been in a regular practice for a while, and the same goes for sustaining the attention to do anything, count breaths, read pages, etc. So, patience, discipline, forgiveness… those are things I recommend.
Also, in terms of synthesizing your learning, consider concept mapping, rigorous outlining, or freewriting (or something else that you know works for you) as a complementary task. I try to create concept maps of what I read so that I can see where there are gaps. I also freewrite my answers to questions to see where I might lack knowledge. After reading this thread, I’m going to give outlining a try, too.
Here’s an example of a map I made about how neuro/biofeedback works, and one about persuasive technology. Yesterday I set a timer and freewrote for 30 minutes about creating an app that utilized “calm technology” to see if I understood how to apply the theory to something practical. I collect writing prompts in Omnifocus so I can visit the list and grab one when I need it. Right now I have 27 really solid prompts I can take on in various ways on any given day.
Most of though, remember that learning is hard and also something you can’t not do. It’s as frustrating as it is rewarding, and there are far worse things you could be doing with your time.
Good luck!