Welcome to the forums and thanks for joining us.
Please use your hands while driving :)
I'm sorry that you're struggling with your first job. I want you to know that everything you are going through is totally and absolutely normal.
My first job was in retail too and dealing with the computer system and the codes overwhelmed me like no tomorrow. I felt like there was enormous pressure from customers, I was going too slow, bothering everyone and would never get anything right. But - yep, like a car, it becomes easier.
I have no doubt that everyone who you're working with (and working with tonight) will have felt the exact same at their very first job or their very first shift. It's downright scary.
When you couldn't scoop the gelato yesterday, that didn't make you useless. That make you a person who hadn't yet mastered the art of scooping gelato! I bet none of us could probably do it easily too unless we've worked in a gelato store! They make it look easy, but it's just lots of practice.
The other thing to know is that you worked on the till and served milkshakes. You were still helpful! The store still needed you and you were still there for them - even with all your anxiety, and as a person with massive anxiety I think that's something to be proud of.
I hope this helps somewhat!