I think I've overstimulated my sympathetic nervous system by repeated stage fright, and now my body overreacts to everything. How can I reverse this?

Mindfullness Therapy. Sympathetic overdrive evokes fear, anger, & general uneasiness. Better to dominate with parasympathetic-related emotions of relaxation, digestion, orgasm, sleep, and appropriate bladder and bowel evacuations. Meditation, relaxation therapies, yoga, & biofeedback are some means to this end. Students learn to use mind to control body. Read relaxation response by dr. Benson & see heartmath.Com.
Try doing yoga, Meditation, hypnosis, sleeping adequately, exercising regularly, positive thinking, humor etc best wishes !