My mom was a "smiler" (Joy Addict) and, in therapy, I learned about this curious coping skill that people learn in early childhood from parents and other powerful role models to deal with sensitive or uncomfortable events & feelings. I suppose one way to break the habit is to become aware of the reflex BEFORE it happens and then create a mental program to remind myself not to react that way or drop the reaction the moment I realize I've gone there - pretty much like changing any habit. IMO, the problem with early childhood conditioning is that it's very, very difficult to change or stop because much of it is connected to SURVIVAL instincts and will continue to ferociously operate to protect us from harm or danger. That automatic "smile" is like a life boat and it won't let you drown without a fight!
good luck,
jim