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 be! I also find it is absolutely necessary to have people who will be brutally honest with you. We all know the ones who will say the sweet lies that soothe us when we are upset or make us feel better when we are down. We also know the not so popular ones who will tell us the bitter truth. These people are your compass - give weight to their opinions. We are not always the most objective judges of ourselves! To have friends, at school or at work, who can remind you of all your positive attributes and help you walk away with your self confidence intact, is the biggest blessing.
Hold the mirror up, remove the rose-tinted glasses and take a hard look at yourself, warts and all. If what you see is what you wanted to see, take a deep breath and tell yourself you are every bit as good as you thought you were! If the image is getting a little tarnished or mutating into someone else, take note of what needs to be fixed and get started on that right away. Remember, you are not as good or as bad as everyone says you are. Don't let circumstances drag you down. Just stay true to yourself and the values that you hold dear. If who you're with or where you are is toxic to your self-confidence and self worth, MOVE. Get out of there pronto.
You are all you've got. Value yourself. Surround yourself with positive people who will help you be a better person and value the person you are right now.
After all,there is only one you. Never was anyone like you before and never will be ever again. That's a pretty huge responsibility. And I believe you're up to it :)