I have had it happen and yes I was SO sure of myself that it definitely made me want to second guess all of my knowledge about life and decisions. (<<These (decisions) are major 'things' though) I thought "what else have I been wrong about" and "now I am not so sure I can make these decisions in the future."...and "maybe because I am Bi-Polar I make all the wrong decisions or come to the wrong conclusions, unlike the way other people do." It was really difficult for me. This was about a year ago. But I decided I can't let "it" (<the situation where I was so sure about that I turned out to be wrong about) hold me back and slowly moved on because if I don't have confidence in my decisions then no one else will either. (btw, I am also aware that there is a stigma attached to being Bi-polar, so everyone is afraid to disclose it. And this is true for me...I do not tell anyone I have it except for family members. AND I am not saying you have it!!) I wish you luck in your dilemma and I'm sure everything will work out. I hope my experience helps you in some way.
Almost all the time. But that's normal, most teenagers always think they are right :P Most of the time I don't go through second-guessing everything I ever did, but you make a mistake, fix it, and move on. Sometimes I over-think the situation and Then go through over thinking everything I've ever done, but that's only if someone egged it on..