It's hard to admit that most of the adversity in my life, most of my "failures" and problems, have been caused by my own beliefs. I've always believed that I'm not as good as other people, that others have much more going for them, that I don't deserve certain positive things, that I'm unlovable, that I have to work harder than others in order to succeed. These are beliefs that were instilled in me, though, not beliefs that are intrinsic parts of who I am. I was taught these beliefs by parents, peers, teachers, etc.--and I believed that I should pay attention to their lessons. My life has changed dramatically for the positive since I've recognized these beliefs for exactly what they are: wrong and harmful. They still hang on, to a certain extent, but for the most part they're not longer a defining force in my life--who I am is much different than I who I used to think I was. For that matter, who I was was much different than who I believed I was--and that was one thing that led to many, many problems.
One of the hardest expressions of self-assertiveness is challenging your limiting beliefs.