How do I design complex high-performance circuits?

You can design most circuits now, and that is real command of a hard discipline. Where you get defeated is the high-performance work, where every small effect you could once ignore suddenly matters and fights you. That difficulty is not a sign you have hit your ceiling. It is a sign you have reached the part where the physics stops being forgiving. At this level, the trouble is usually that everything interacts, and you cannot hold it all in your head at once. The people who do this well lean hard on tools, on simulation, and on breaking the design into blocks they can verify one at a time. They do not out-think the complexity by raw effort. They contain it, piece by piece. Take the next hard design and isolate the single part that worries you most. Simulate just that, understand just that, before you assemble the whole. Master one demanding block at a time and the impossible design becomes a set of hard but doable ones. You are not lacking talent. You are learning to fight complexity the way the best do, in pieces.