I agree with what was said above, that a cashier in a fast-food restaurant is a very high stress environment, and going to be anxiety provoking!

There are plenty of jobs that you could look into that are lower stress and don't involve customer contact. Before going to university I worked as a warehouse picker (you get the order, take the items off the shelves, pack it and ship it) and also did data entry, both of which are low stress, little face to face contact and plenty of opportunity to take a break when you need. There are lots of other options, they just happen to be places I've worked. Good luck finding something that suits you!