Why can I build what I'm given but not decide the structure myself?

You said it yourself: give you the design, and you build it well. Ask you to choose the design, and something in you locks. That is not a skill gap. You already have the skill. What you lack is permission — and no one is going to hand you that. The terror is not about the decision itself. It is about the silence after it, when there is no one else to blame if it fails. You have spent years working inside someone else's judgment. Now the job asks you to work inside your own, and you have not practiced that. This will not be solved by studying more patterns or reading more architecture books. You already know more than you use. It is solved by making small structural calls, alone, on record, until the silence stops sounding like danger.