"You cannot have too high an opinion of yourself because the world will always strive to correct you. The only thing most people hate more than success is self-confidence-a warning signal that you might be a success soon. This is not what they teach you in Sunday School, but it happens to be true: at any evidence that you might be a success, the envious will do every-thing in their power to destroy you. Therefore, there is no chance at all that a high self-esteem will go unchallenged; it will be challenged on all sides, daily. On the other hand, if you have a low opinion of yourself, nobody will ever correct it. You will have it for life unless you correct it yourself."
The entire essay, "Making It As a Writer," is well worth reading.