Alexander Technique will definitely help your posture, which helps good breathing.
Wikipedia has interesting info on yawning.
I used to calibrate air quality sensors, and found that when I was running the Zero Air (no CO2, no humidity, mostly just nitrogen, I think) I would have problems breathing because the CO2 in the room was too low. It took me a bit to work it out, but I would feel a little strange, and have to force myself to breathe while at my desk. If I went for a walk, or did some pushups in the stairwell, I'd be good for another couple of hours. Apparently, the urge to breathe is triggered by high CO2 levels, not by lack of oxygen.
So... ummm... Get some plants? Get some fresh air? Do some exercise?
This seems like a good thing to see an ear/nose/throat specialist.
Journal on depression/anxiety
Curing IBS-C with Vitamin C and magnesium citrate.