Post-interview jitters

Just got done with my first ever job interview. I thought I would feel more confident afterward, not less.

I did fine. I had all of my answers prepared. I was ready for any question I was asked. I had questions for my interviewer. It being my first interview, I'd say I did pretty well. And it's an even better job than I expected - innovative, changing, groundbreaking.

However, I feel like I didn't sell myself. And since we're talking about an advertising job, that's certainly not a good thing. I had plenty of opportunities to assure my interviewer that I'm the perfect person for this job because everything - all of the qualities and expectations - applied to me. I thrive in a fast-paced environment. I'm organized and efficient. I love to learn, and I learn fast.

So then why do I feel like I failed some test that I didn't even know I was taking?