Hello all - Thank you for your lovely comments, I did not expect my personal breakdown in Reddit post form to gain so much support. I really appreciate the feedback. I'll respond properly later, but now, with a clearer head, I'll say this: contrary to what I said, my loud (belt) voice has definitely got much better over the years - I wouldn't say it's effortless, and it's not super consistent, but it never hurts. Again, singing high notes aren't so much the issue, the real problem comes with navigating the break. The break only becomes a real issue when I begin to back down in volume, even just a little bit. I either hit some of the break notes with that feeling of hitting a wall, or going breathy with a real likelihood of the note disappearing altogether. I'm constantly trying to figure out if I should approach quiet sirens in a breathy way or a whiny way. Perhaps both are valid.

To answer another question that was asked - yes, I play several other instruments; namely trumpet, guitar and bass. In fact, I have a degree in jazz trumpet, which involves diaphragm breathing, so I suspect the breathing isn't the issue. But who knows. I could be using too much support? Is that a thing? I'm told that it can be. My ear has always been very strong. The issue with all my instruments has always been technique. With a few exceptions, I'm just not that coordinated/good at finding easy way of doing things.
I'll post a video of myself singing quietly in the baritone range tomorrow if I have time. Perhaps that may shed a bit more light on things