ask for more training. or work another shift with less customers until you start finding your rhythm. ask your co-workers for help to guide you.
if it comes to it, and they do fire you, that's just tough luck. maybe look for another job in retail, or a warehouse job. maybe look for a business job like receptionist, administrative assistance. those don't require a lot of training or education to do. some companies will hire and train for inside sales, business development, appointment setters, call centers, customer service, or other similar positions.