I played golf for the first time two weekends ago and played extremely well. The reason? I was relaxed. I used to golf a fair amount and was always focused on the outcome and on getting better. I practiced a great deal and really wanted that work I put in to translate in to better scores on the course. In reality what happened was that while my game improved technically the pressure I put on myself for results created tension in my game which hurt my golf score. Why did I play well this last time I played? simply because I was not worried about the result as I had not been playing and was just out to enjoy myself and to catch up with a good friend.
I have been finding the same problem I had with golf before creep in to my squash game, tension. I work really hard on improving my game and it is working as I am a much better player but I do get too focused on the result. This becomes more of a factor in matches where I know I am playing someone of equal or greater ability and league or tournament matches.
The key to overcoming this as I have found from my recent rounds of golf is being relaxed. Work hard during practice to improve your game. Definitely play hard in your matches as well but don't worry about trying to win every point, every game, and every match. Relax and play the best shot you can at that moment, get back in position by getting back to the 'T', relax and repeat. Let the results take care of themselves.