Why can't I keep my pace up for a whole shift?

You pick fast and clean at the start. By the end you are slower, sloppier, and spent. That is not a flaw in your character. It is arithmetic — you spent a sprinter's effort on a job that needs a long walk. Most people never question this. They treat their first hour as the standard and judge every later hour against it, then call themselves weak when they fall short. The standard was wrong, not you. The skill that matters here is pacing, and pacing is something you build, not something you either have or lack. If the exhaustion follows you home, or you cannot recover between shifts no matter what you try, that is worth raising with someone outside this page — a manager, a doctor, someone who can see what you cannot see about your own limits.