except that it feels harder than it used to be.

I have the same feeling, and it's driving me bonkers. And I'm not even studying anything new or difficult. I revise French and Italian, and I seem to grow more insecure all the time. It must be this time in life, having accumulated lots of facts etc. When I studied those things in school, it was a build-up of information, and I kind of sucked it in and it stuck. Now it seems like digging in a big container of information, and I can't remember what's right and wrong. It really bothers me.