How do I cope with the constant vigilance of a job where the enemy only has to win once?

You can defend against almost anything. But you carry a brutal math: you have to be right every time, and the attacker only has to be right once. That never lets you rest, and over years it grinds on the nervous system. What you are feeling is not paranoia. It is the honest weight of the job. You cannot carry that alone, and you were never meant to. Security is a team and a system, not a lone guard who never sleeps. When a breach comes, and in this field they come, it is a failure of many layers, not proof you personally failed. Sharing the weight is not weakness; it is how the defense actually holds. Build real rest into your life, not as a reward but as maintenance. A vigilant mind that never recovers gets slower, not sharper. Trust your systems and your team to hold the line while you step back. You defend better rested than you ever will running on empty.