I've the same problem now and then. Usually it's when it's 0:1 and and then it's 2:1 and I'm losing.. I get ticked off when I missed one stupid little shot. Squash is super mental. It's why we play it instead of racquetball.
The other thing.. is we all need to practice on our own more often. We need to get to where getting back to the T as well as ball control and hitting the ball is natural.. like muscle memory.
But honestly, without seeing your games.. it's impossible to say what is causing you to win the first couple and lose the rest.
But my two bet guesses would be 1) your brain is getting psyched out, 2) you are running out of energy or a little of both i.e. you're running out of energy and can't think straight.
Another is that you're doing the same moves all the time and your opponent is catching on. Another is you're warmed up but your opponent isn't.
1) STAMINA: Ride your bicycle for at least 10 miles., at least 15 miles an hour the entire way. If you can't do that.. I would suggest getting to the point to where you can. And then do better.
2) SKILL / CONTROL: Keep practicing AT LEAST 45 minutes before you play a game.. hitting the ball down the side wall.. try to get the ball to bounce in the serve box every time. And then behind.. and then in front. Then rotate. Start practicing with your backhand on this drill first.. if you can't do 2.. keep trying until you get five. Then go to your forehand side and do five.
If you play with a double yellow or yellow, and you're still having a tough time with this drill, try practicing with a blue dot ball. It will allow you to use less strength, and work more on your ball placement.. your swing needs to be slower.. so it helps you think about how you're moving and swinging. it's easier. I start with the blue ball any time I am having problems with a drill using the double yellow. Usually by the end of the session, I'm on the double yellow and kickin bootie :)
Go at least once a week to the squash court just to practice on your own. The other days that you play games, practice on those side walls alone for at least 20 minutes before your games...
Additional tips.. heard them a million times before maybe.. but if you haven't.. you should :)
Three things I ALWAYS tell myself
1) Eye on the ball - the front wall ain't a gonna move. so stop looking at it ;)
2) Keep the ball on the wall (away from the middle of the court) The wall is either the front wall, back wall or usually the side walls. I actually take masking tape and section off the middle of the court.. I stay inside the masking taped area. the ball stays outside it.
3) Get to the TEE. Constantly work on immediately getting back to the T after you hit the ball.
And if you need to pace yourself.. then you need to get a bit more fit (hey.. i'm in that boat!) or not play as long - because there's only one pace to squash! ;)