"If someone gives me a deadline, I can make it. If I set my own deadline, I don't make it. Why?" Have you heard that question before?
It seems to me, when someone gives you a job, they believe you can do it. They are validating your ability by the assignment. When you give yourself a job, if you don't believe in yourself, you won't meet the deadline. You cannot, or will not, validate yourself. Why?
Does humility mean self-doubt? I don't think so, but you will have to figure that one out on your own. In fact, only you can answer the question, "Am I able to do the job(s) God believes I am able to do?"