Then you've probably never yet really faced a true life-or-death adrenalized situation, because the tunnel vision is part of the physiological effects of the stress hormones.
Let me explain that a bit...
Several people have done a lot of research into the physiological effects of combat or fighting stress. Dave Grossman has collected and done a lot with this; his website is at
www.killology.com (though he's joining with some others for a new site
www.warriorsciencegroup.com). Basically, as the body responds to situations, hormones do various things (shut down capillaries, pool blood into the vital organs, and more) to prepare the body to survive.
Properly formatted training can simulate and innoculate you against some of these responses -- but you almost have to be in situations that are a very near approximation of the real event to achieve all of it. (Note that I'm summarizing material that could -- and does -- fill several books here.) We each respond to different situations differently. For some people, their first sparring match gets pretty close, but over time, they get accustomed to it and sparring becomes a comfortable experience. And the innoculation effect is somewhat specific to the stressor; comfort in a fist fight situation doesn't automatically equal comfort in a gun fight or a car crash.
There are training exercises that can be done - with reasonable safety - to prepare and innoculate you. See, for example, the book
Training at the Speed of Life by Kenneth Murray, or some of the exercises described by Rory Miller in
Meditations on Violence.
And, remember, that bravery is not the same as not feeling fear. Bravery is acting IN SPITE of feeling fear. Chesty Puller, Audie Murphy, and any others you care to name all felt fear. They simply refused to allow their fear to control their actions.