"Of all the cants which are canted in this canting world…the cant of criticism is the most tormenting!" Laurence Sterne
While I hope we have all rid ourselves of the phrase, "I can't," criticism greatly impedes our progress as tennis players. It is only harmful when we do this to ourselves. However, you say, "Should I ignore my mistakes?" "Should I not care when I err?" No, in fact I would suggest the opposite. Tennis is a difficult sport. Errors are unavoidable consequences of playing. We must learn to gain an honest, critical eye towards our game. That is we must evaluate objectively, as opposed to simply finding faults in ourselves. While embracing errors can be a positive experience, negative criticism can make one's development in tennis a slow, torturous process or worse.
"Your problem is you bring in the critical factor before the lyric factor has had a chance to express itself." Joseph Campbell.
Although written to combat writer's block, this statement holds true for tennis. Criticizing ideas before they have made it to paper prevents one from ever starting. One must get their ideas out first before returning with a critical eye. I often hear in a lesson, "Oh, I'll never do this," or "I always screw that up." These criticisms are roadblocks towards improvement. You must allow yourself to make mistakes in order to improve. Mistakes are valuable learning experiences if one can only view them as such. Constant pressure of failure restricts growth. Improvement will come when one goes out and gives his/her all, noting mistakes, but discarding the negative view towards them. After assessing the reasons and possible solutions to the errors, it is time to head back out for more.
"Sometimes you wake up. Sometimes the fall kills you. And sometimes when you fall you fly." Neil Gaiman (on the fear of falling in dreams.)
Criticism can lead to similar options to one's tennis development. A person can bring such a great deal of criticism to his/her game that playing ceases to be a pleasure. This often ends in the player quitting tennis, forgetting whatever dream that had attracted him/her to the game in the first place. A second state sees a player falling short of his/her aspiration, due to continual criticism. This person still plays, but is dead to enjoyment and improvement with tennis. The third state is a player whom has let go of self-criticism. This person has learned to evaluate errors as important, even enjoyable learning experiences. This leads to constant renewed interest and love for the game as one improves. I believe we are all capable of flying. Be kind to yourselves.