Best wishes Veinglory.
YMMV, but for me, when I was stuck on finding sources for specific parts, it helped to get the university librarian involved. I seem to recall you teaching at a university? Even if not, they may be helpful to anyone.
What surprised me was not just the help I received earning my Ph.D., but also later as an Adjunct Professor.
And, the years from student to professor brought about something new to the school: a subject-specific librarian. As Adjunct, I was editing an article for academic publication and could not find an original citation. I finally asked the linguistics librarian (my field), and within a couple of hours he had several options for me. He was so excited to have helped. He said it made his day. Wow, I was so honored for his help that saved me hours! So now instead of feeling like a heel for bothering them, since I thought I should already remember (or whatever I am thinking), I now pick up the phone or send an email when necessary. Don't know if this would help, but just throwing it out there in case.
Also, might I suggest trying a different outline plan than what you are used to? I am doing that as well with good results so far. I am writing my first novel, but still trying to finish my academic textbook that is currently in scrivener on my Mac (and I use Storyist on my iPad Mini). Anyway, I just found an amazing tool to organize and check outline for my novel, that has helped me unmuddle being stuck in the middle.
Well, I like it so much that I plan to do this to check out structure of my nonfiction text too, just to confirm that I am only including parts that teachers will need (it's a text for a subset of teachers), and also to see that I am doing it in a concise but complete manner.
I described how I am using it in a thread on Basic Writing recently.
A link to the free template is here, in case it helps you or someone too.
http://www.fantasyscroll.com/master-outlining-and-tracking-tool-for-novels/
I hate spreadsheets usually, but this one is color coded and so simple to use. And while for most of it I followed the intended order, I also went in a different order sometimes by listing sections (well scenes in the novel) and going backward to see where they fit, or where they should be moved.
Anyway, these are some tools I am finding helpful to get moving forward, where I was stuck.
ETA: and for nonfiction, I plan to finish plugging in all info into this template based on what I have outlined and written in Scrivener, and then be able to use this outline as part of my proposal for the chapters not yet written (of course I know much more is involved in such a proposal, but this outline should help show the big picture and also planned details nicely). Of course, the page comparing to 3 Act Structure is not relevant for nonficiton, and I will label chapters with sections inside chapters (so use that terminology instead of scenes for them to see). But it is just an idea that sounds promising.