Want to quit after 3 months

It’s about my second month at the teller line and I already want to quit. I feel completely overwhelmed and am always paranoid that I’m going to do something wrong. Plus, I made a huge mistake yesterday where I accidentally gave a business customer too much money when he wanted a change order. I got a call from my manager after I left and I felt completely stupid. I’m wondering if this is normal? I’m so stressed out going to work every day.