I am too stupid for my job?

I can relate. I quit a cashier job after only like three weeks because I kept making mistakes, I had basically ZERO training and they threw me on the cash register as if I was somehow automatically supposed to know everything. I worked with all females, and they would look at me like I was an idiot. Definitely felt like bullying a couple of times. They were all older than me and were on the job much longer than me, but they kept making me do everything and sort of felt like they were throwing me the "hot potato" when it came to all the difficult work just because I was new there. Hated it, and that was my very first job too, so it made me scared to get another job because I figured every work place was probably like that.
Making mistakes at a job happens pretty often I would assume, if you're pretty new. It just goes to show you the lack of training they gave you, which is their fault. If you aren't totally confident in what you're doing, the training wasn't good enough.

This is SO stressful. I need the money but some time has gone by and this is very hard. I really don't think my personality just isn't right for the job. I've really tried to change but I can't. This is the same thing that happened on my last job. It's hard to sleep through the night sometimes, I'm tense and nervous even when I'm not at work. In the perfect world I would quit but I really can't because I need the money. They can't fire me but at this point it would be a relief. They can't really fire me now but I guess it could happen. At this point, there is no other option besides sit there and deal with it so I'm basically just venting.