Why can I handle my part but not the whole plan?

You can speak with confidence in your own field. Ask you a question there, you answer it. But hand you the whole operation — every department, every dependency, every person's job at once — and something in you locks up. You call this a planning weakness. It is not. This is a scope problem, not a competence problem. Your mind is doing exactly what minds do when asked to hold too much: it stalls. You have quietly made yourself responsible for things that belong to others — their decisions, their timelines, their judgment. No one assigned you that. You assigned it to yourself, probably out of fear that if you don't carry it, no one will. You are working with others. That means the operation does not live in your head alone — it was never meant to. Your task is smaller and harder than running the whole thing: know your piece cold, and let the rest belong to the people whose job it is.