It is practically impossible to keep your calm in such situations when you know deep inside that your stutter will kick in anytime now. I completely understand what you went through, and if it makes you feel any better, I recently went through a similar experience and it took me a while to forget what happened and start afresh. At this point, I'm absolutely sure that I will stutter if there is even a single person listening to or looking at me while speaking and there is nothing I can do to avoid that. Forget the past, do your best in the present, and wish for a better future.