Cashier problems

Im 16 working as a cashier and people are so incredibly rude to me for no reason and I don’t understand. I’m nothing but nice to people even when they bash me and I want to be able to stand up for myself. I’m just not sure how without getting fired.. and by people being rude I don’t mean getting mad because I rung up something incorrectly. I mean like one day I had an early shift so I was obviously tired and I forgot to scan someone’s water under their cart and she called me autistic because of it. I’m basically bullied by people I don’t even know and sometimes I just want to quit