It sounds like you're psyching yourself out. and you're doing it to protect yourself. and I use humor much the same way. Anyway, it is scary and nervous but that's good. you dont want to be comfortable and not move anywhere. I had a retail stocking job in a drug store, and despite stuttering (coudl be quite severe at times), I had a good time. My coworkers didnt really seem to mind and most of my customer interactions were so short it wasnt a big deal. if I really couldnt speak, i'd just walk them to where the item was. most customers were nice and it was all good. I guess my biggest advice is see the good. We may have a natural tendency to see the bad, but dont focus on that. remind yourself about the good and you'll at least be facing in the right direction.