This is a very good question. I have some experience with this; I find that as situations get more stressful, I become more and more calm and try to deal with it. I think it's a combination of a personality trait (my mother recently reminded me that I was the calmest of all her children) and a certain amount of self training. I used to remind myself (and others) that worry and panic do nothing to help the situation and that times of stress, more than ever, require a rational response. Eventually, the rational response mechanism discards emotional or stress based reactions as sub-optimal and you search for better ones. I know, easier said than done, but better said than never.
