I just thought of something I have been trying to phrase pithily. It has to do with my feeling that much of the time I am ruled by fear. That state degrades my ability to deal with the current moment in a rational way. It demoralizes me, and just plain feels bad.
Holding your breath is an unconscious reaction to fear. The protective instinct of fear is to become as quiet as possible, so as not to give away one's location. How many times in your life has becoming quiet actually helped you evade something you feared? How many times, on the other hand, has a clear, calm mind helped to succeed in doing something? Which leads me to pithiness.
Don't hold your breath. You need plenty of oxygen for your brain to work properly.
Notice when you are holding your breath, and make use of the awareness that fear is affecting you. Practice letting go of instinctive fear, so that the appropriate parts of your brain-mind will be freed up to deal with the situation. I'm not saying to deny that you are afraid of something, but rather, make an intention not to operate unconsciously in fear. Express the fear in words, and let the rational knowledge of the specific fearful thought affect the rational evaluation of what needs to be done.
I can say from experience that this is easier to understand intellectually, than to do.
For example, imagine you are driving to work, and thinking about various things that are happening at work. You think about what you need to do when you get there. Something that might turn out badly occurs to you, and you are now driving down the road holding your breath. You might not hold it completely, as you would underwater, but you don't take breaths that are as deep (and loud) as you would if you were relaxed.
Every time you think of something negative, check your breathing. Take a deep breath. Yawning out loud is another good thing to do, if you are alone, or in a relaxed setting.