Yep, it almost feels counterintuitive to relax to play faster, but everybody says that's the key. I still tense up.
Interesting about the breathing. If you want, try breathing in slowly for two bars, out for three bars maybe, depending of course on the speed of the tune. :) Yoga practitioners often do the four-fold breath: breath in for a slow count of four, hold for four, breath out for eight. or some version of the same, in for four, out for eight, hold for four ...
I haven't tried it, but if you did this at half the speed of the music you were playing, it should synchronize nicely as long as you're playing something in 4/4.
About tensing up: If any of you have never had a professional massage done, yes, professional! *wink*, do it. I thought I wouldn't be able to even relax enough to benefit, but after three times now, I can definitely say it helped me realize what "totally relaxed" is supposed to feel like, albeit a bit sore too. So much of my tension is unconscious it's bizarre.
But, yogis would say the same thing about breathing: most of us do it too little, too shalow, etc.
Chris