You can prevent stress in the midst of difficult situations by exerting any control you have over the stressor. In my book, Power Over Stress, I offer many practical ways to deal with stressful situations before they turn into stress.
Here is one way to defuse a stressor. You anticipate and plan.
Let us say that, this weekend, you have to meet your husband who has just moved out of the home demanding a divorce. You know the encounter will be very stressful. What you do is to anticipate the stressful nature of the meeting and deal with it in advance.
See yourself in the room where the meeting will take place. See him blaming and criticizing you but see yourself breathing deeply and being calm and collected and responding in a very mature and responsible way never inflaming the situation. Visualize yourself going thru the meeting and at the end feeling energetic, calm and healthy.
Program these feelings by visualization and when you actually get into the situation, it will be as if you have experienced it before and the brain will oblige you by making the rehearsal a reality.