Getting rejected from University of Chicago and Northwestern. After studying for the SATs each weekend for over 6 hours, setting up various community projects, raising my GPA to over 3.8, it was a kick in the stomach, when I found out I probably won't be able to accepted because I'm an international student. Even worse was the letter I got from Northwestern which a part read "you were not rejected because of your financial standing, and your country citizenship" when I know that they have a separate pile for international students, who cannot afford to pay full tuition.
Another horrible moment was when I destroyed a friendship between me and a friend, because I had feelings for her, and chose to bring it up selfishly and at the wrong time. We didn't talk for a year, but she closed the gap, when I was too selfish to. I look back, and I think what a jackass I was.