Having self-doubts is a wonderful spur to growth and improvement and, indeed, the worst performers in my experience tend to be those who believe they have already "arrived" and who therefore have a tendency to coast and stagnate while the best are those who always see a need to strive to "fill holes" in their repertoire. At some point, one gains authentic self-confidence based on a record of achievement but even then the need to continually examine one's weaknesses is critical to sustained growth.