Be your biggest fan!
Self-confidence. The most fragile thing on earth. I have seen the most self-confident, capable people develop cracks in their self-image, doubting their ability to handle the simplest of tasks. Sadly, our image of ourselves and our abilities is tied to the way others perceive us. So if you get passed over for a well-deserved promotion time and again, or see others who are clearly far less capable than you outshine you, its only natural that in time you will begin to question your image of yourself. I have seen this disease strike across all ages and occupations - teenage students, seasoned professionals, fresh graduates... The best way to take this on, I find, is to have a set of criteria by which you measure yourself. These need to be very specific and clear, so you don't leave room for delusion! Every now & then measure yourself by this yardstick and do the necessary course-correction as required. After all, you are the only one who knows what you are shaping yourself to ...