In a genuinely stressful situation I believe it is as you say - we're not likely to be able to control ourselves and choose reactions. But in our lives there are lots of "repeated stressful situations", these are situations that we face from time to time, maybe daily, situations of similar kind (like performance in front of a big audience, necessity to get acquainted with strangers or climb to a great hight, etc). In these cases I believe we can choose our response through time by making this situation from stressful to an ordinary and familiar one. I believe this could be achieved through practice and continuous deliberate putting oneself in such situations, making the fear go away and learning which reaction brings us more benefit.