You often hear about “being in the zone” and that refers to your level of focus but it can also refer to how aggressive you are.
I played my best squash when I was angry. Not angry enough to verbalize i.e. shout but very close.
Personally, I dislike the displays of aggression from any sportperson but in the heat of battle it’s hard not to let them overflow.
However, what I really want you to start doing is playing from the first point with the same internal aggression as you do when the score is 8-8.
In fact, a good practice is to start at 8-8 for a few games every now and then. It will teach you to not waste any points.
I was often too relaxed at the beginning of matches because I knew there was a long way to go, but I wish now that I had fought with the same intensity in the first few points as I did in the last few.
I can’t exactly say how you develop that skill, but start by finding out what your mental approach and state of mind is when you have played your best.
As I mentioned above, for me it was when I was angry but also had a clear gameplan that I could stick to.
Remember, nobody won a tournament for being too relaxed and friendly on court. I am not saying be rude and nasty to your opponent though.
Sure, you can be nice for a few points but you need to be motivated for 99% the rest of the time.
What I am saying is generate that fire in your belly from the first point.
Imagine a best of five game match where each game was one point only.
How hard would you try for those 5 points?
Play THAT hard for EVERY point!