In my experience, a phobia like this is akin to hang-gliding - it looks and feels far more scary thinking about it than it is actually doing it.
You can easily work yourself up itno a frenzy of anticipation, and then be fine right through.
If you concentrate on breathing deeply - in through your nose and out through your mouth, that has two effects - it ensures that you are fully oxygenated which will prevent you hyperventilating, and secondly, the concentration needed to do that will take your mind off your fear and keep you calm.
Good luck.