Jesus, can't imagine how upset you were back then. I was wondering why that customer was so rude to you and his behaviours were very aggressive and impolite. I hope you are getting better. I didn't work as a janitor so I cannot offer very professional advices sorry but I suggest you to talk about your stuttering and that you didn't deliberately frighten that customer away and whatnots to have your supervisor's sympathy. If the outcome was bad then let's say you are so unlucky to gotta deal with a obstinate supervisor.