Guys are dancing around it a little bit.
STOP worrying about your results. It's very hard to let go, but it's the key to enjoying the game again.
When your are learning/practicing how to stay in the moment and not worry about the results, a great example to emulate is Maria Sharapova.
See how she gets the ball and still goes to the back fence to pick at her strings? Do you know what she's thinking? She's mentally reviewing the previous point. If she has any negative emotions she doesn't turn back to the court until she's processed them. Then she turns back to the court, does a little skip-step and encourages herself to focus on the point at hand. By the time she starts dribbling the ball to serve all thoughts of the previous point are gone. She knows as well as any other top player that it's a waste of mental resources to dwell on the past point or carry negative emotions into the next point.
Being tense, worried about how you'll perform, worried about how you'll be perceived for not performing...it's all a negative feedback loop. Those emotions prevent you from executing your strokes and then that cycle spirals out of control if you let it. Being loose, self-confident, and unconcerned with the results is a positive feedback loop.
Personally, I had to play and lose so much that I basically became numb to it. But when I resigned myself to losing, I discovered a freedom to enjoy the process and once I experienced that freedom, I actually started to win!
Another thing to remember before beating yourself up too much is that even pros make unforced errors, and lose close to half the points played, even when they win!