Just thinking out loud (so to speak)...Without a drill sergeant standing over your head stressing you out, ask yourself what stress is and how can you duplicate it. Exercise -- work-out to elevate your heart-rate and then do whatever it is you want to accomplish while under stress. If you are literally running for your life, then wind-sprints might be a good exercise. Run hard to elevate your heart-rate and tire your muscles and then a nice relaxing walk back to the starting point where you do it again (and again and again). Condition yourself to that physical stress but without a mental stress component.

There's a book titled, "Mental Mechanics of Shooting, How to Stay Calm at the Center" which is not a plug for the book (hard to find) but for one technique on how to calm the fear/panic. Just do whatever it is while calm and when you do it for real, the calm will return when you do it for real -- theoretically. In the book the idea is to only practice shooting while calm. Add an elevated heart-rate but no external threat (drill sergeant) so you are actually practicing under physical stress, but not necessarily mental stress. It seems that if the authors theory holds, the act of doing should bring back the calm. You are returning to something familiar and your body responds.

OTOH, if you are in a panic when you practice, the act of doing may bring back the panic -- theoretically.

FWIW, $.02