You are describing some pretty dry subject matter. That sort of material can be hard to get engaged with when you're not attending lectures that help put an outline in your mind and gets you doing some relevant note-taking. I bet that's more the problem than any change in your native capacities. Emotional ups and downs take a toll on motivation more than they affect cognitive capability - IMHO. At least that's been my experience.

My academic work involved a fair amount of pretty dry subject matter. You can't sit down and just read a chapter, like you would a short-story in an English Lit. anthology. The texts that go with fundamentals courses are actually a lot more engaging that the technical tomes that treat advanced subject matter.

It's hard to focus your attention on advanced, dry, technical stuff in the absence of a concrete, practical goal. Part of the brain rebels and says, "So what . . . who cares?" The dutiful part of your mind says, "I should be interested in this." Realistically though, you just aren't. Someday you can be - given a context that supplies some motivation. You might want to leave some of this matter for later when you might be taking an actual course that requires it. In the past, you have wrapped your brain around difficult concepts. But that was in the context of hearing a professor lecture on the stuff, having classroom discussions, sharing notes with peers, etc. Why do you suppose it's near impossible to learn a second language without being immersed in a country where it is spoken - no matter how many courses you've taken in that language? The human brain is economical. It tends to not retain what it has no compelling need for.

To keep up with your academic area of interest, look more to journal articles that aren't excessively scholarly. Read the kind of journals that are accessible to non-specialists - like Psychology Today. Those can be a springboard to delve into more technical stuff.

Right now you need to either take a course or get a job . . . or finish kicking back, if you've chosen to take a break. In the context, eventually, of passing a course or performing well in a work-environment, you'll have motivation to study specific literature that is relevant to what you're doing.