We all love to buy stuff. New rackets, new strings, new shoes, in our hope to become better players. Maybe that extra kilo in your stringing will make you hit with more control and get rid of some mistakes?
You try a new racket. You are inspired. You concentrate, you are powered by the NEW and you are hitting tremendously. You play one of the best sessions of tennis ever and the rackets gets all the credit. You know who should get that credit? You! You are the one who concentrated more, moved better, and played with conviction, not the racket. The racket was only the tool that inspired you to reach that level.
The next time you hit with the same racket you might not be so inspired. The thing is not NEW anymore. You expect the racket to lift you, but instead you are not playing so well and you are starting to doubt it. The racket might not be your saviour anymore, it actually feels strange to you, foreign to your hand and swing.
The racket might indeed be better or worse than your previous one, but in most cases (if you have decent tennis gear) it is not in the racket, it is in you. If you come to the court ready to give it your all, you will play well, no matter if you are playing with a frying pan.
The one thing that can help your game the most is your feet. If you get up to a good level on your footwork you can really excel and get that extra time to hit your shots the way you want them. Footwork is the key to good tennis, to all your shots, and that is something you should focus on if you REALLY want to improve.
A new tennis racket is a great thing and can lift your game, if only slightly. Footwork (and of course technique) is where the real improvement can be made. Think about that the next time you're on the court.