oh ((((Biiv))) You're not falling, you're moving forward. Yes, new things are often scary. But like you said, it's a happy time (even though you don't feel it.) I'd be worried if you weren't nervous!

Let's go over this "the best I can be" idea, ok?

Are you saying there are times that you don't do your best? (Not that we all don't have those times.)
But, I mean, even now, aren't you doing your best, considering all you are doing and your mind is trying to think about?
Of course you are! It's that you are comparing your best right now, to your "all time best." IMO. And that's not allowed! You compare your best to what you are actually capable of right now, and what you are actually doing, right now.

Don't expect everyone there to expect you to hit the ground running. Take your time and seat in a bit. Get organized, asked the others questions (it makes them feel smart, and appreciated too!)

Lack of confidence goes with every new job, imo. Gird yourself up. Try thinking about another time when you began a new job... and how you felt the first day, and how you felt the last day. It will be that way for this too (except I hope you don't have a last day for a very long time!)

TC. I believe you will do your best and that will be just fine!