Relaxing to improve

I had an epiphany in the pool yesterday.  I realised that when I try to swim fast I start to tense up, my stroke shortens becoming less strong, and I end up being slower and quickly running out of breath.  To avoid this, my focus has become any tension in my body.  I try to concentrate on relaxing this tension and trying to utilise each stroke as much as I can while that arm is under water.  

I wonder how many things in life this could apply to.  For example, if I'm trying to cook dinner in a hurry I stop doing things efficiently.  The pasta is ready long before the sauce and vegetables are.  Or I forget a vital step like adding salt to the bread dough.  When I used to perform to people on either the violin or piano, I would think too hard and lose my place in the music, making a massive glitch I wouldn't normally have made had I been playing at home alone without the pressure of an audience.  Or my bow would start to shake and my left hand fingers would play out of tune.  I'm forever trying to get through to my adult students that being relaxed is the key to most of their technical issues.  I think I need to start taking my own advice.