How do I handle the weight of designing things people's lives depend on?

That weight you feel is a sign you understand what you do. People will live inside your decisions, walk across them, trust them without ever knowing your name. Feeling the gravity of that is not a flaw. It is what makes you careful, and careful is exactly what the work needs. But a weight you carry every second will crush you, and a crushed mind makes worse decisions, not better ones. The way through is not to feel less. It is to put the weight where it belongs: in your methods, your codes, your checks, your margins. You do not hold safety by worrying about it. You hold it by doing the work right and then trusting the work. So build the habit of finishing. When a design is done to your standard, mark it done in your own mind and let it rest. Carry the care into the next one. You were given a serious job because you can bear it. Bear it, but do not let it bury you.