Hi Albert! Confidence really develops over time. It comes from studying/rehearsing, so you get good at what you do. It comes from performing (such as going to an interview) and having both failures and successes. When you truly did the best that you could, there is no reason to be ashamed. And, even if you made mistakes, you will grow from them, so that is a good thing! You might want to consider getting into activities that require you to "perform." This could be something like track & field, acting, debate team, etc.
Letting you in on a few secrets. Most people who act confident really are not. They are just better prepared to deal with a "disaster." They are the ones who, if they were running a race, and tripped over their own feet and fell down, would stand up, take a majestic bow, and then continue to run. Secondly, when interviews go bad, it's sometimes the other person's fault! Not everyone is an expert at interviewing - perhaps it is their first time conducting interviews! When I teach classes (to jobseekers), I think I project confidence because I know my material and speak very candidly. I don't try to talk like a bureaucrat. People can tell that I am speaking from the heart, and they respect that.
Now, this might surprise you. I used to be a police officer. When I showed up, people listened. The uniform made that happen. When I retired, I was terrified! I had zero self confidence! But when I say it will come over time, this is what I mean. When I started teaching, I was scared. What if... What if the computer failed in the middle of the presentation? Yup, that has since happened. I lived through it. What if I had unruly people in my class? Yup, that happened too. Dealt with it. The more you experience success, the more confident you will be.
In the meantime, don't be too hard on yourself. Nobody has a perfect interview. If you did "okay," consider that a success! The power of positive thinking will carry you a long way!!
As to your peers, don't put yourself down in a bad way, but sometimes it is okay to joke about yourself in a light-hearted way when you do something goofy, you know?
Feel free to stay in touch! Kim