I think this happens to everyone. No one instinctively knows how to do this. In addition, you learn by seeing existing designs and critiquing them (positively

I think this happens to everyone. No one instinctively knows how to do this. In addition, you learn by seeing existing designs and critiquing them (positively or negatively... knowing what to avoid is almost more important IMHO). I would even say that most people working in software can only fill in the blanks, they absolutely cannot start from scratch (which is in truth very hard).