A bad shift ends. How you treated the customers during it is what follows you.

The difficulty of a day is temporary and your conduct inside it is not. The rush ends, the broken machine gets fixed, the shift finishes, and what remains is what people saw you do while it was all going wrong. Stress is an explanation, not a licence. Nobody keeps score of how hard your Tuesday was, but they remember being snapped at.

On the next bad shift, pick one thing to hold steady whatever happens: greeting people properly, or not sighing, or apologizing for the wait. Let the rest be as rough as it needs to be. Afterwards, let the day be over instead of rehearsing it.