The story of Moses striking the rock is only a short seven verses of the Torah. And yet it is one of the most well-known stories of all. (Try it yourself: Write a story in seven sentences that will be remembered by much of the world three and a half millennia hence. You've got to be impressed.)
Getting back to the point, however, what exactly is the nature of Moses' mistake? Does he get angry, does he lose his head for a moment, does he get impatient?
We need not conjecture. The Torah spells it out for us in simple Hebrew: Moses, the lawgiver, the one who brought the Ten Plagues to Egypt, the one who split the Sea of Reeds and produced manna from Heaven, "did not have enough faith in God" (Numbers 20:12).
Now whatever "Moses not have enough faith in God" may mean (and it requires some serious explanation), one thing is clear - this is no little mistake. It's serious stuff. For the Torah to say this about Moses, and for God to 'punish' him by prohibiting his entry into the land of Israel, he must have done something pretty bad.
King Solomon tells us in Proverbs (24:16), "A righteous man falls seven times and rises. An evil man falls but once." Every one of us, even Moses, transgresses. Every one of us makes mistakes. Often big mistakes. But the issue is not whether you make mistakes, it's how you deal with the mistakes you make.
Steal once and you are not a thief. You are a good person who has stolen. Steal a number of times and you are still not a thief. You are a good person who steals. Identify with the act of stealing; see yourself as a thief - only then do you become a thief. A righteous man might steal, but he will try to change who he is. He may steal again and try to stop himself again. He may steal many times and "rise," as Solomon says. He only becomes the evil man when he stays down; when he says that he cannot rise; when he gives up on ever being good and stops trying. By so doing, he creates a self-fulfilling prophecy.
Moses made a major mistake. In fact, he made a few major mistakes. And yet, he is considered the greatest man in Jewish history: the lawgiver, the teacher of all of Israel, the man who spoke to God "face to face."
God does not demand, or even request, perfection. He merely asks that we strive toward that goal. There are many stumbling blocks along the way. And we are bound to stumble, and stumble again. But it's crucial that we see them as setbacks, not defining moments.
We all make mistakes. It's part of being human. But don't identify with those mistakes. Don't see yourself as being incapable of moving past what you have done. If you are a good person who makes mistakes, you will always rise again no matter how low you fall. If, however, you see yourself as a "bad" person - i.e. since you are "a thief," you might as well steal. In that case, you'll stay down when you fall. And that's the biggest mistake of all.