Serious question: how do I never self harm again regardless of any extreme stress I experience?

If I am under extreme stress and my whole world is falling apart in an instant one day like it did on October 26 how do I not automatically seriously self harm under impulse and also relapse at the same time? As soon as the moment hit me that I was away from the source of extreme stress I lost control of my what I was doing and went to self harm. I never want this to happen again. What should I do to prevent this from happening again especially if the source of the extreme stress is something I can’t prevent and totally crushes my self worth and self esteem while in withdrawal (No PMO) from 3 years of PMO addiction?