I sometimes catch myself not breathing. You’d think that a person would actually know when they stopped breathing but believe me, it can slip right on by when your brain is busy doing other things. A vivid reminder on how powerful the mind it. It happens when I’ve been either intensely concentrating on something specific (aka obsessing), or keeping up with my monkey-brain and its 50 thoughts jumping from branch to branch as fast as they can. 
Refocusing attention to my breathing gives that brain only one thing to think about, and breaking this cycle of building anxiety provides relief. To simplify things, I don’t really think of it as watching my breath — which always sounds a bit like watching invisible air whooshing about — but rather paying attention to my stomach as it goes up with each inhale and down with each exhale. 
Remembering is the hard part, but I’m getting better. Stopping to watch my belly instantly calms the contortions that my brain is putting itself through. When you feel overwhelmed, STOP for a minute and pay attention to what you’ve been taking for granted your whole life. 
   
No need to get in special poses in special places for long periods of time. When your mind is a runaway train — taking off all by itself at full speed — a couple minutes of breath focus gets it back on track and brakes the accelerating stress. And breathing automatically slows, providing a little respite for your blood pressure. Your body will thank you.
Thich Nhat Hnh, in his book Peace is Every Step, goes a step further and emphasizes smiling because it helps you “approach the day with gentleness and understanding.” He recommends mentally reciting “Breathing in, I calm my body. Breathing out, I smile.” I haven’t gotten so far as to smile with each breath, but I can recommend the health benefits and the power of breath breaks. They bring you back to the present moment whenever your mind’s been flitting between what happened yesterday and what’s going to happen next. You get to reboot, like passing “Go” each time you lap the monopoly board with $200 in your pocket.