You could have been fired on the spot for that. After all, you've had problems in the past and you were getting numerous complaints from the customers. Your manager gave you a second chance.
Listen, I know you're young. But the economy sucks. Getting fired from a job can haunt you for a while in terms of finding another one. And in a crappy economy, people who have bad reputations - e. g. reputations for not following directions, poor customer service, etc. - are less likely to be hired.
You are getting a steady paycheck. Love this job or hate it, only you can decide whether you are going to pay attention and do it correctly and make a good reputation for yourself. A good reputation means you're kept when others are fired. It means that you get a good recommendation from your supervisor.
Basically, decide whether you want to remain employed, because I am certain that there are plenty of people in your area who would love the chance to be able to serve customers properly.