M.J.
Our daughter had this same fear. What worked, and this sounds goofy, was to make fun of the toilet. Went to one and did a funny routine along the lines of, "Are you a mean potty? I hope you're not so loud. We hate loud ones." I would hit the toilet seat, look at the whole thing funny and say, "Well, it's pretty quiet, isn't it? Let's give it a try." (then thoroughly wash my hands, yuck) I honestly think my goofy routine stunned our daughter. She laughed and since that time hasn't had issues preventing her from using toilets outside our house. She still hates the self-flushing ones, but the terrified fear is over.