I know this feeling all too well. I landed such a great job two+ years ago (still working there now). I feel stunted by my inability to speak at times or use the words I would like (or even pretending not to remember something I can’t say), it kills me. If the words came out, I would be considered the most thoughtful, innovative and articulate on the team — and instead I usually sound like a babbling idiot. I am sorry for what you are going through. As a previous commenter mentioned, I hope it went much better than you perceived and that you get the job! Send a follow-up email, pull out all the stops. Let us know how it shakes out.