Just Got First Job, But Why Have I Changed?

Before I got my first job, I wanted nothing more than to start working, so that I could not only get my first job experience, but earn money to support my family. I then got hired for my first (and current) job at a retail store. It started out okay, and I was always bright eyed and excited. But now, I want nothing more than to quit. I don't know what happened to me to destroy my psyche in the past six months. But now, with each day that goes by, I've begun to hate my life and dread the thought of getting up early for work in the morning. My first two months, I was bright eyed and bushy tailed, had a very chill manager who always said to us "only do what you can" but it seemed our only communications were work-related. The next four months roll around, and we have a brand new manager, because my old manager got her own store, and the new manager wants things done completely different. As time goes on, I started hearing conversations about me and other co-workers, I hear employees talk bad about employees, managers talking bad about employees, and sometimes it's just to whoever walks in the building, like the customers. I even caught my managers talking about me, but when I go to talk to confront them about it, they start to treat me like I'm some kind of fragile little baby. The customers seem to be worse now than ever before, they went from super duper nice, to so rude and condescending, to the point of calling me names berating me for no good reason. I've been taking meal brakes at earlier times, having nightmares about various things, I start crying and having mood swings for no good reason, and I've lost my touch with customer service. Now, nothing in my life brings me joy anymore, either in or out of work, I'm much more exhausted than I'd ever been before. My emotions swing from one end to the other, and I also noticed I am snapping at family members. I sometimes catch myself seeing horrible visions of terrible events that never happened to me. My sleep schedule and my diet have suffered dramatically, and I go from losing one pound to gaining six in the course of like two week, then the whole cycle goes around again. I've become very angry and depressed, and now I have all this emotional baggage that I don't know what to do with.

What's happened to me? Has my work environment changed, or have I? Why do I feel so toxic on the inside, when before I felt a yearning and inner peace? My life has fallen apart into shambles and I don't know if it's because of this retail position or not. Is my job mentally destroying me, or is it something else that I haven't seen yet?