Easy for you to say "just be confident". I failed the process twice before getting in, and I found it very difficult. Maybe I was not confident enough the first time, but I really found that I was not controlling the steps of the interview. And it spiraled out quickly in a tough interrogation...
I don't think that giving the best shot is enough - that's why I was not selected the first time around. The year I got in, my interview went so well. I prepared it so well that I was in full control of the questions. And same thing for the group assessment. There is a specific training to it I believe. A total different story.
But at the end I agree, this is not the end of the world if one does not get in!