I know the feeling - working on (undocumented) legacy systems is a nightmare for so many reasons. But not all companies are like this. It sounds like you need

I know the feeling - working on (undocumented) legacy systems is a nightmare for so many reasons. But not all companies are like this. It sounds like you need to a) quit and find work some place with less legacy/debt (ideally a startup), or b) ask to be moved to another department that has less of that kind of work (if possible.) Continuing with work like this will, as you've found out, burn you out sooner or later.