Hy JULES, how are you doing!?
Congrats on going further on the interview!!! It's your first victory, and you should celebrate!
Anyway, I do understand how you feel... and it's how most of us feel during an interview. We all get scared, and insecure about ourselves, and afraid of saying too much or not as much.
But first I'd like to tell you that, at least I have that feeling that I overexplain stuff, or I was talking to much, and right when I leave I remember something that I should have said!!!
Because we create so much expectations, and expecting to do a perfect interview is unrealistic... so drop all the super high expectations and aim to be yourself, no matter what.
Be honest all the time, and don't try to look like you are a different person, you got here being yourself (also because once you are hired, you would have to keep up with that image you've created, and that will stress you out).
One thing I do is, before the interview, when I am getting there (like, when I park my car, or my bike) I do a quick deep breathing exercise. Than I repeat to myself: "You can do it, you're no better or worse than anybody, so just go there and show them".
Hope that helps!