Stephanie, I’ll assume that this behavior is only occurring with you and not with the other guardian of the horse. You will need to do a bit of “research” to see if you can identify what is triggering the fear responses. You might want ask a friend to videotape a few of the interactions so that you can review your behavior and the resulting behavior from your mare.
I have observed several horse/rider conflicts that developed from one or two unfortunate interactions. Your horse may be reacting when you place the saddle on her back because one time the saddle dropped down hard, causing her discomfort. She then reacted the next time in anticipation of that same discomfort. Her reaction might have stopped you, at least initially, from putting the saddle on her back, which then reinforced the behavior.
Be sure to look back through my column and read the advice I’ve recently given others about body language—understanding how your body language is perceived by your horse might help you in your discovery.