Here's my take on the whole MCAT thing. It sucks how many people really can't tell how well they performed based on how they felt walking out of the test. One person feels terrible then aces it, another felt just as bad and truly bombs it. For me, I am the type of person that always knows how well I did on tests. Yes, even standardized tests. In April, I walked away feeling like I gave it my best shot, and that if my studying paid off, I'd get between a 31 and a 34 on the real deal. I actually scored in that range.
But driving home, I remembered all these questions that I was totally confused about, but I know that I picked the answers to the best of my abilities. Which means using logic, deduction, and the foundation you've built in the basic sciences. I even went so far as to write down all the questions I might have gotten wrong, then further tormenting myself by guessing my scores if I indeed got all the questionable ones wrong. But you're not going to get all those wrong. And I agree with the other posts, torturing yourself isn't going to help.
Here's how I knew when to panic. I took the test in an earlier administration and felt terrible walking away. I would look at these BASIC questions that I knew I should know how to answer, but got so freaked out that I ended up guessing randomly because I was too nervous to think straight. For future testers, if this is happening to you, I suggest voiding the test. Voided tests don't get recorded on your AMCAS application.
But this past April, I was thinking straight, dealt with some hard questions, but really felt I gave it my best shot. That's when you can start to feel better coming out.
Also, remember that the experimental questions are wacko. I felt like I knew exactly which ones were experimental, and if I let myself feel bad about them, it would only make me feel worse. Don't do that to yourself. I guessed that about 10 of those questions in the PS and BS sections were experimental, and the MCAT admin told me I was right. So everyone should walk away feeling a little defeated because we had hard questions that were tough to answer. Some perhaps worded to badly to answer. Those practice tests don't have those experimental questions.
Sorry for the long response. Others have different experiences, but for me, just knowing myself and what I capable of gave me some relief.