I'm in the same boat. Started a little over a year and a half ago. When I started I was in my junior year of college working 20 hours a week and having 3 semesters of 15 credit hours for my degree. Just graduated in May and I'm hanging on by a shred. You're not alone in thinking is. Many of your coworkers have the same experience as you but they won't speak publicly about it. Truth is retail banking is shitty. Customers aren't going to change. Pressure from management isn't going to change. The only thing you can change is what you allow to affect you.

In my own experience, the only way to cope in this role is to simply STOP CARING about customers and management. You have to do it for your own mental health. It's kind of hard because the training for the role revolves around customer empathy and solutions. Obviously be friendly and give a good experience, but don't derive your value from customer experience surveys and activity goals or whatever they call them at your bank. No, you can't have a deep personal relationship with every customer and do everything flawlessly despite management thinking likewise.

> my assistant manager constantly jumps on those little mistakes and fussing at me all day

Don't let them get to you. At the end of the day, your manager is very replaceable and their opinions of you mean next to nothing in the long run. Have a decent enough relationship with them to where you could use them as a reference, but don't let someone that hardly has any more experience than you tear you down. Slow down, don't be be pressured to keep the line moving quickly, and do the best you can.