A visceral fear response is difficult to control, in that it is largely based on the release of adrenaline and similar internal hormones into the bloodstream. Once they've been set loose, they can't quickly be neutralized.

About the only way I know to improve one's chances of avoiding a surge of fear in an emergency is to imagine oneself in such a situation, in as much detail as possible. You may not be able to duplicate all the conditions of a real emergency in advance, for example you wouldn't set a grease fire in your kitchen to learn how to put one out. But mental practice in dealing with a dangerous event does help to make it seem more familiar and manageable, even if you've never actually experienced it before.

As Minx and WD would probably point out, and rightly so, putting your whole self into the effort of imagination, by physically going through all of the steps you'd take during the actual event, is the most complete and most effective way to prepare oneself mentally, even if it is not always the most practical way. To extend my previous example, most of us will never actually confront a 'test' fire and discharge a fire extinguisher as a form of practice, so that we could see the flames, smell the smoke and feel the discharge at firsthand, but if we did, we'd be better prepared than those who only pointed the nozzle at a non-existent fire and imagined pulling the trigger.