Please don't feel silly - every job interview is just as important and just as difficult as the other. I am proud of you for getting this far. That is a big deal. It means they saw something in you already that is worth pursuing. It means to they already have some confidence invested in you. Run with that.

Regardless of what happens, you will have gained some experience to your advantage. So it is cool what happens either way.

I think we all can say we are proud of you and rooting that things go well.

The best of luck then!

As for keeping your cool during the process. It really helps if you have something to hold onto. Practise some mindfulness - how do your feet feel against the floor? Sense the feeling of that pressure on the floor from your feet to your hips and above to your head. How does it feel touching that object you are holding onto? What are the surroundings like about you. Is there anything you can find comfort in.

Answer the questions, provide examples as needed, and try not to ramble. While they will be looking for how quick you think on your feet that is not the deal breaker so take a little time to breathe and gather your wits about you between questions. This will demonstrate some confidence.