How do I cope with big decisions resting on uncertain assumptions?

You can model almost anything now, and that mastery has shown you something unsettling. Underneath every model sit assumptions you know are uncertain. And on those uncertain guesses, real money and real decisions rest. That weight is heavy to carry. But notice this. You see the uncertainty clearly, and that is exactly what makes you good. The dangerous person is the one who forgets the assumptions are guesses. Your discomfort is not a flaw. It is honesty, and it is the thing that keeps your work grounded. You cannot remove the uncertainty. You can be clear about it. When you present your work, state the key assumptions plainly and show what changes if they are wrong. Let others see the ground you are standing on. That honesty is your real contribution, more than any single number.