How to deal with stress at my job?

I'm trying to figure out how to cope at work when I start crying or becoming irritable. I deal with the public and I have no clue why but when a customer gets mad or raises their voice or calls me names , I kind of shut down. I start to cry or I get very angry. At this point in time , I am starting to become unmotivated and wanting to avoid people as much as possible. I'm thinking its probably best to quit. I'm starting to become someone I don't recognize. I get very angry at a drop of a hat over very little things. I also start thinking too much and start crying. My sister doesn't want me to quit. She helped me get the job and she really stuck her neck out for me. She keeps saying I need to suck it, stop complaining and not let random people get me upset when I'm getting paid to do this. She keeps saying if I quit, I'm just running away from the problem and I'm going to continue to face the same issue over and over. I don't know what to do, I was to be happy but on the other hand I don't want to lose a decent paying job and face the same thing over and over again.