In the corporate world, you can't rise above the level of confidence you have in yourself. If you have a BSc and limit yourself to applying for a gateman's job, well, that's what you'll get. But if you apply to a manager's role and don't accept anything thing less, you'll be turned down multiple times but you are bound to meet an employer who will grant you that managerial role because of your confidence. So it's very important that you believe in yourself, and to a high degree.
Don't just believe in your ability to do what you have done before or passed an intensive certification exam on, but also believe in your ability to do what you haven't done or tried before. Be more optimistic and enterprising. Worry more about success that failure; think of what to do to succeed at the big task your over-confidence has landed you and not the bitter consequences of failure. Be willing to learn on the job rather than before the job. Take up roles you are not fully qualified for but are willing to work an extra mile at in getting fully qualified and delivering the results expected of you. Let the level of your belief in yourself correspond to your loftiest aim and not to highest degree/certificate.
When you compare yourself to the best or those who are much better/qualified than you, pay more attention to how they started and not how incompetent they make you look. Bill Gates didn't learn programming by not making mistakes; he once got banned from using his school's computer because of the damage he once caused on it due to his incompetence and curiosity. Steve Jobs acted immature and incompetently for the most part of his early corporate career and had to be fired from the company he started. Thomas Edison was most proud of his mistakes and believed that a great success is founded on numerous failures. What they all have in common is not verifiable competence at what they attempted but huge self-belief that drove them to attempt all they attempted.
Even Mark Twain said, 'The secret of getting ahead is getting started." Putting yourself forward with little regard to your qualification is the secret of getting ahead in life. You get from life only as much as you demand of it. And the only way you can demand great things from life is to have a great self-belief. To have an earth moving confidence in yourself. To be willing to attempt what you are not qualified for and to think more in terms of "What if I succeed?" than "What if I fail?"
You have to believe in yourself!