How do I stay alert on security without burning out or getting paranoid?

Your work asks you to expect trouble, and that is a strange thing to ask of a mind. Do it too little and you fail at the job. Do it wrong and the vigilance never switches off, hardening into paranoia that sees threats everywhere and a body that never gets to rest. You are trying to find the line between real watchfulness and that grinding, always-braced state. It exists. The difference is not how alert you are. It is whether you can also stand down. True vigilance is like a muscle. It works because it can relax. It tightens when there is reason and releases when there is not. Paranoia is that same muscle locked in a permanent clench, exhausting itself on threats that are not there. The goal is not less watchfulness. It is watchfulness that knows when it is off duty. So practice the release on purpose, because the alertness comes easily and the letting go does not. When your shift ends, mark it with a small ritual, and tell yourself plainly that the watch is handed off and you are off duty now. Train the standing down as deliberately as you trained the standing guard. That is what keeps vigilance sharp for years instead of burning it out in one.