You can be quite the mental taskmaster, like the stern high-school teacher who is never pleased and holds impossibly high standards. At the same moment you are the student, one who either works slavishly hard to succeed in class, earning every point and developing mental discipline and muscle, or perhaps one who gives up early on, feeling as if the battle is lost before it's even begun, so what would be the point of trying? In this case, guilt and a deteriorating self-image set in.
You're prone to negative thinking, but it doesn't have to be that way. It's good to be skeptical; no one can take advantage of you. But that dour expression and furrow in your brow have got to go! Consciously keeping your mind open and flexible, exposing yourself to new ideas and conversing with people who hold different opinions than you will do a lot to keep your mind flexible and keep you teachable. Periodic bouts of mild depression should not be a cause for alarm; this is how your mind and soul renew themselves. You turn mental challenges into strengths through discipline and conscious effort.