This is because there is ignorance, and ignorance doesn't want you to think you have the power to overcome it. We call ignorance, Mara. Personification of ignorance is often put into the frame of Mara. Mara doesn't want you to uncover your deepest potential because Mara will not have any power over you anymore, keep you in ignorance. One of the best ways for Mara to keep its power is for you to believe in how terrible you are and how little potential you have to overcome it. This is what Mara does, Mara likes to use this attitude of reflecting about your negative things. That's why the Buddha gave us so many confidence inspiring objects, he realized that we're all very good at reflecting on how terrible we are, and this is the power of Mara. So in order to overcome this we need a lot of practice in reflecting on confidence inspiring objects, to actually point out to Mara that the image that he or she is projecting is just an image, and idea. It's a bit like there's an object, there's a 360 degree circle around the object and we're stuck on one side and we're viewing the object from this side, and we believe that this view of the object is the only view there is. We have to actually walk around the circle and point out continually that this image that we believed is not real. It's not the totality of the object. And from this, the idea of negative self-image as being "me" tends to disappear, little by little, it takes some time.