I am suffering a similar level of pain right now. I wrote about ~18,000 lines of Z80 assembly language (w/ ~5000 lines of comments) in the past few months. I can feel the pace of development slowly grinding down, because each new feature or refactoring requires the manual testing of the previous features that have already been added to the app. In other words, my development process has become O(N^2).
I feel like I will be forced to create some sort of testing framework at some point, to have any hope of extending the app beyond a certain level of complexity. But I currently have no idea how that can be done in an assembly language project. So I am trying to delay the inevitable for as long as possible.
(The dev environment is the TI-83+ and TI-84+ series of calculators using the Z80 processor, if you are curious.)
https://mos.datatra.sh/guide/unit-testing.html