I'm not a confident person, except in job interviews. Then my confidence just bursts out of me and that's my recommendation (as much as it may not seem like one if you're not confident).

But there's no good reason NOT to be confident. You're clearly qualified, or you wouldn't have the interview in the first place. They're interested and more importantly they need YOU or you wouldn't be here. There is literally no answer you don't know, because the most correct answer to a question you literally don't know is just that.

"I don't know/have that answer/have a solution, but I will find out."

It's so much easier when you realize that the above isn't just an acceptable answer, it's a good answer. It's the best answer you can possibly give. When I am evaluating someone there are two things I personally look for above all else. First, personality. Can I stand to be locked in a room with you for the majority of 8 hours? Cause I can teach you anything you don't know for this position, but I can't teach you a new personality. Second, which ties into this point, is if you find yourself in an unfamiliar position, how are you going to handle it? Do you stop at the first roadblock and then ask for help? Will you attempt to work beyond what you know and try things to see if you can discover solutions you're not familiar with? Or do you rush headlong and refuse to accept it's beyond your abilities and make a bigger mess than there was initially?

Speak loudly and clearly. Make eye contact with everyone. That room is yours. You are literally the most important in the room in that moment. They wouldn't waste their time otherwise. They know you're nervous, you're meeting people whose sole job in that moment is to judge you. Everyone has some inherit fear in such a scenario. So how do you handle pressure?

Finally, embrace it. You can learn about yourself as much as they can. This is a challenge and a perfect opportunity to learn from.