It's not uncommon to come out of the test and feel like you bombed it. I personally felt a lot of anxiety throughout most of my test for Step 1 and Step 2 CK. I felt way less confident about my answer choices than I did during my UW blocks, which led me to mark twice as many times as I usually did during my practice tests. I left the exam counting up the number of stupid mistakes that I made. I ended up scoring higher than my practice test averages on both Step 1 and Step 2 CK.
At this point it's all out of your control, so there is no use in working yourself up until you see your score. Just realize that a lot of people walk out of the test feeling terrible and end up scoring a lot higher than they expected (myself included).