Get yourself a notebook and begin to jot down your accomplishments EVERYDAY. These may include little mundane accomplishments like making up your bed in the morning or brushing your teeth. Get into the habit of recognizing and acknowledging your achievements. No matter how small they may seem.
It does not matter what the accomplishment is. What is important is that it is acknowledged, it is recognized. And in doing so, that begins to reinforce your confidence within self that you actually DO accomplish and that you actually DO incorporate abilities to accomplish what you want, and it stops that automatic association that you cannot or that you will not.