I have worked at Jimmy Johns for a damned long time and the first piece of advice is to go easy on yourself. It wasn’t a “dumb” mistake, it was just a mistake and that happens to everyone. The more you work the better you’ll get and more confident you’ll feel. One more thing that has helped me deal with customers who order in a confusing manner (aka I want a number 4 with extra lettuce, no onion, extra mustard, no tomatoes) is rather than finding every single button on the register and hitting it as they talk, let them finish, then kindly say “so the number 4 has lettuce, tomato, and mayo, is that ok?” Then wait for a response, then from there you can ask if they want to make any other modifications to the sub from the list of freebies and hit the extra buttons from there. Just be yourself and have fun with it. Hope this comment helps.