So, in the end I could only keep up my self-imposed language learning regimen for about two weeks, and then gave up. I am starting to suspect I just don't have the stamina to study something linguistic every day, I have to go more slowly than that. It's possible I could benefit from more focused learning too.
I think I'm going to boil it down to just Mandarin (ZH) and another language, German (DE), for now.
I've kept learning more Mandarin frequently enough, so that at least has been going fine, but I haven't done anything with German this year yet. I was trying to remember the German I learned that summer 4 years ago, I noticed I remember the personal pronouns but I don't remember some other very basic things like how to conjugate haben
correctly, in the present tense! Basically, I'm almost starting from scratch. I think I'll work through April Wilson's German Quickly: A Grammar for Reading German
(36 chapters, 420 pages).
My aim with German is mostly just to get to read Gabelentz's Chinese grammar and use some of the great resources for Latin there are in that language. No need to speak it or write it.
Oh, and I haven't abandoned Biblical Hebrew (HBO), but that doesn't take much time. My answers for Lesson 5 are coming this weekend.