quitting a job after two weeks?

Hi everyone,
I need urgent help please. So i started my first ever job working at pret in an airport, now i know this was stupid of me to do. I feel like i’m being exploited, they haven’t trained me at all and they know this is my first job. I wouldn’t have an issue with that if it weren’t for the fact that everyone’s always complaining about how i mess up, even on my first day people were complaining about me. Let me clarify as week by “messing up” i mean maybe getting an order wrong or not leaving a customer to grab another’s drink. The reason i feel so exploited is due to the fact that i’m contracted for 16hrs a week but they’re scheduling me for 30hrs, am i even allowed to refuse to work the extra hours? Anyways i’m planning to quit after the second week, is it okay if i call and tell them i quit? (and this is the most important part- could someone let me know what i should say). Do you guys think they’ll pay me, or will they withhold my pay because i quit early. The job is only a seasonal job for 3 months, and before u say “ just stick it out” i physically can not, im genuinely afraid of ending up in hospital if i keep it up, i don’t even have time to eat or sleep just work and anxiety 24/7. Sorry this is so long, but pleaseeee help me!!