Why do big productions defeat me when small ones don't?

You can direct. That is settled — stop doubting it. What happens on the large production is not a failure of talent. It is a failure of method, and the method was always going to break once the thing got big enough. On a small production you hold it all in your head and your hands, and it works, because there is little enough to hold. On a large one you try to do the same thing, and you cannot, because the thing has grown past what one person's attention can cover. You are not failing at directing. You are failing at a task you never actually had to do before — governing a structure instead of making every choice yourself. This is not comforting, and it should not be. It means the fix is not more effort on your part. It is a different grip. Loosen it where it does not belong. Tighten it where it does.