What does that voice at the back of your mind keep telling you? What makes you uncomfortable or ashamed of yourself? This could be anything from mistakes to regrets, disappointment, friends at school, or a past traumatic or negative experience. Whatever is making you feel unworthy, ashamed, or inferior, identify it, give it a title, and write it down. Then, tear or burn these written pieces to start feeling positive on those points.
This exercise is not meant to bring you down. It is meant to make you aware of the problems you are dealing with, and empower you to move past them. It gives you this feeling of "letting go". I tried it and it worked(s).