You are NOT stupid. It sounds like you've been at this job for a short period of time, so it's very normal to feel like you don't know everything about the job yet. I hated working in retail, my coworkers were so not helpful and tended to take advantage of me too - but I was also too unassertive to say anything. You have a bachelor's degree and you're feeling inferior to your sister for having one extra year of college than you? Bachelor's degrees take four years, on average, master's degrees take a total of five years so that's one extra year on top of your bachelor's. That's nothing to feel inferior about. I don't even have a degree yet, so I would loove to have a bachelor's right now. I'm working on my associates (ugh).
Anyway, everyone says it takes six months to feel comfortable at a job. Six months. Not one month.