If there is ever a reason for young people to go to school, this serves as a wonderful example. Jobs like the one you have are entry level jobs. Hours often stink, managers play games, and you have little power in either skills or education to bargain with.

I would not quit this job. I would experience the difficult task of sitting down with this person and see where she thinks you are lacking. However, in your position, I would not think it hard to figure it out. You run a register and put together peoples orders, essentially. So, she is probably saying you are too slow in one of those areas. You either are or are not slow.
If you are not, find another job cause you are not going to win this fight.

If you are, work on it and you should see a positive change. If not, well, see paragraph above.