It was like that for me, but I found I’d not concentrated enough on the very first exercises and was trying to work through the course too quickly.
I’m not saying that the same applies to you.
I just wanted to share my experience with you because since going back to the start and really focusing more on getting the very basics as right as I could, I have started to get my (limited) range back and am sounding much better.
I also find playing easier so I don’t want to go back to working much harder for limited results.
The breath support issue I totally relate to. I think the point is …. Reaching for notes with your stomach!
Instead I’m trying to find them by slowly narrowing the aperture corners after letting just air through the pipe and raising the tongue a little.
Supporting the air a little for higher notes but only after I have heard the pitch faintly sound.
Anyway, whatever you do I hope it goes well.
You are obviously dedicated to learning to get that frustrated!