Customer service/retail can be a stressful environment because you never know who will walk in the door, and you are supposed to be chatty and helpful and friendly and all that good stuff. Some customers are just awful, but this is everywhere I suppose; we just can't avoid them. If you want to teach you should totally go for it. Also, that little encounter with the customer and your manager, that is sexual harassment! You just don't do that to people. It makes me so angry that this person said these things right in front of you. For those situations I always wish I have a good comeback, but it only comes way later.
I was wondering about something, is this your first job? Starting a new job is always stressful business actually, even if it is not your first. Most everyone I talk to about getting along in their jobs mentions that they end up crying in the bathroom at first (I experienced this as well). It can be overwhelming trying to learn the ropes and getting along with everyone. The people you work with do sound like an awful bunch
People do give us a hard time for being introverted and quiet, don't they? And they will try to make it their project to get you out of your shell. Ahhh. Hang in there *hugs*
