I had a similar experience after getting hit by a car while crossing the street. A friend who saw me and came running over actually thought I was in shock or something, because he couldn't believe that I was taking it so calmly. The adrenalin also helped, of course - I didn't feel any pain at all until nearly half an hour later. The only emotional reaction I can remember was extreme annoyance at the breaking of my glasses.

Oddly enough, I fall into the same category as the post author. I don't think I'm very good at reacting swiftly, and I absorb information and handle that sort of testing very well. But I do generally manage to keep my head. I suspect this is a pretty recent development, which may affect my perception of the matter.

I've found one useful heuristic. If I periodically remind myself to keep cool and reevaluate the current situation, I generally get good results. Slow-reaction types tend to quickly make a bunch of assumptions when thrust into a situation calling for swift action, and weeding out the erroneous ones is very helpful.