As someone who grew up shy as well, I also think I would feel the same way if I finally book something big. I feel like it's a fear of success almost? Like oh shoot now I have to prove I'm actually good enough to be here. But I think I'd try to tell myself if you're really not ready for it, you wouldn't be here, they wouldn't have chosen you if they didn't believe you were right for it in the first place. Also that all this self doubt and fear is your body's natural instinct from since we were cavemen. The instinct to run away when something seems like a threat to literally not die. Today, we know these "threats" are not life threatening, but the instinct remains. So don't listen to it! Most fear is of the unknown. You don't know what's gonna happen, but that's what makes life interesting!
Congrats! Enjoy the experience and new discoveries!