There are only a few days a year when you will walk on a tennis court and all seems to go well. There are many more instances when you are struggling with one or more shots and the ability to put a good point together. It is extremely important to learn how to be competitive (and even successful) when you are not playing as well as you would like.
The first step to achieving this is to recognize that everyone has bad days, including the best players in the world. Don't try to figure out why this is happening or get frustrated with your lack of ability to hit your best shots. Try to set some basic goals like hitting three or four balls in a row or just hitting more shots solid by watching the ball more closely. Many times you will be able to play better as the match goes on and possibly even win the match.
Sometimes it is just going to be "one of those days", but you need to continue to challenge yourself to compete and try and turn things around. Dealing with adversity in a positive way is the sign of a good player. We are all capable of this if we don't let our emotions take over.