How do I handle complex high-stakes design work that overwhelms me?

You can design well, which took years to earn. The complex, high-stakes projects still defeat you not because your skill fell short, but because the pressure and the scale change the whole feel of the work. When more is riding on it, the same hands that work smoothly on smaller jobs can freeze. That is human, not a flaw in your craft. Big work becomes manageable when you stop facing it as one enormous thing. Break it into the smaller design problems it is actually made of, and solve them one at a time the way you already know how. The stakes feel smaller when the piece in front of you is small. Much of what overwhelms you is the whole looming at once, not any single decision within it. On your next big project, name the first small piece and start only there. You do not lack the ability; you have proven that already. You are learning to carry the weight of scale, and you do it by not trying to lift all of it in one breath.