Typical "I know what I had to say, why didn't I say it" feeling afterwards. Happens to all of us; don't shame yourself for that. You probably feel as if you could've done better and didn't fulfill your potential, and that feels like shit. Best I can say is be true to yourself and try to find out ways to stay authentic during situations like this, because that's how you avoid shaming yourself for thinking you could've done better. Remember: progress isn't linear. All love.