Sounds more like the problem is low self confidence not rude customers. Nobody can make you feel dumb if you know you’re not, and you’ve given lots of proof your are not dumb. Your co-workers sound jealous. Look in the mirror everyday and tell yourself what your boss has said, that you are good at your job and the most qualified. This is a temporary situation cause once you graduate you will move on and have a huge collection of stories to entertain your friends with about how rude people were but you did not let them get to you.