I agree with Greg. The first thing is to be 100% confident in your equipement. Try to pay more attention to your surroundings and not on your fear. I can guarantee you that if a shooter came in your fear would suddenly dissappear. try to train yourself to concentrate on other things.
I actaully hate heights but its no big deal for me to get 25 feet up in a stand. while i'm up there i look around constintly looking for movement.