How do I work safely on damaged or unknown high voltage systems?

You work safely with high voltage now, which already means you have real discipline. The damaged and unknown-state systems still frighten you, and that fear is not a flaw to overcome. It is exactly the instinct that keeps you alive. A person who stopped being afraid of an unknown high-voltage system would be the dangerous one. Your fear is doing its job. The way through is not to feel less fear but to channel it into procedure. When the state is unknown, you treat it as live and lethal until you have proven otherwise, every time, no shortcuts. Test, verify, isolate, lock out. The routine is what lets you act steadily even while afraid, because you are trusting the process instead of your nerves. Discipline, not fearlessness, is what mastery actually looks like here. Before the next uncertain job, walk through your verification steps out loud so they are automatic when it counts. Let the fear sharpen your care rather than freeze you. Respect for the danger is not something to grow out of. It is the thing keeping you whole.