The worst thing you can do at this point is to shut down & go silent again. If you do that, then you need to accept the fact that you're going to repeat this type of situation again & again until the day you die. Continue putting yourself outside of your comfort zone like this & it will become less stressful, even to the point of feeling routine if you keep at it. It's hard, but if you can persevere through it & maintain a positive outlook, your confidence will go up, & in turn, the severity of your stutter will decrease. It may sound backwards, but you need to expose yourself to this type of situation as much as you can personally handle if you want to avoid ever feeling this way again.