The stress reaction is triggered by your perception of danger. . . Try to temper your excess emotions. Put the situation in perspective. Learn to moderate your physical reactions to stress. Slow, deep breathing will bring your heart rate and respiration back to normal.
Relaxation techniques can reduce muscle tension.
Electronic biofeedback can help you gain voluntary control over such things as muscle tension, heart rate, and blood pressure. Medications, when prescribed by a physician, can help in the short term in moderating your physical reactions. However, they alone are not the answer. Learning to moderate these reactions on your own is a preferable long-term solution. Build your physical reserves.