How do I design for failures I can't predict without being paralyzed?

You are holding two true things at once: what you build can fail in ways you cannot foresee, and you still have to build it. Too little respect for that danger and you get careless. Too much and you freeze. Finding the narrow path between is one of the deepest parts of your craft, and wrestling with it means you take it seriously. The way through is not certainty, which you will never have, but humility with a method. You cannot predict every failure, but you can assume failure will come and design margin, redundancy, and warning into your work. You plan for the unknown not by knowing it but by refusing to trust your own perfection. That refusal is what keeps people safe. When the weight of it starts to freeze you, turn it into a next action. Ask: what is the most likely way this fails, and what is one thing I can add to catch it? Do that, then ask again. Paralysis comes from facing all the danger at once. Discipline comes from meeting it one safeguard at a time.