Many of the features of your mental set originated in your childhood, before you developed the ability to think as you do now - abstractly. Abstract thinking begins to develop in adolescence and really comes into its own in adulthood. The way you thought as a child is very different from how you think now. If you want to learn more about this, read a book on cognitive development. I prefer to make the point by illustration.
Suppose that I made the following rude remark to you: "You are nothing but a stupid jerk!" At your current age, you might get angry or annoyed with me, but you would be able to figure out that if I call you a jerk, it doesn't make you one. You are able understand that you are not a jerk because I say so, that I am a rude person, and that is my problem, not yours. Being able to think through that kind of situation requires an ability to reason abstractly. You did not have that ability when you were a small child.
Imagine that you are only five years old. Imagine that I said the same rude comment to you: "You are nothing but a stupid jerk!" Being a small child and lacking the ability to reason through this kind of message, you would be apt to believe it and take it completely to heart.
Childhood is a time of mental vulnerability because children lack the abstract reasoning ability of adolescents and adults. Adolescence is the transition period. Some nuisance faulty beliefs about ourselves, others and the world will become established during our childhood years, before the onset of the ability to reason as an adult. Once established, these thinking patterns will not necessarily fall away as our ability to think grows up. Such is the staying power of habits.
As we pass through our teen years, developing abstract reasoning skills, no one takes us aside for a time-out to check out the degree of mental pollution acquired during our mentally vulnerable childhood years. Some of the troublesome thinking patterns and faulty beliefs of childhood will persist in even extremely intelligent people. The good news is that it is never too late to begin the task of mental house-keeping, and helping your thinking to grow up.