If you don't show up to your test, all that happens is that you lose the money you paid to sign up for it. Two days after you miss your test you can sign up for another one for late August or early September. This is not ideal timing for this application cycle, but doing well in several weeks will nonetheless leave you in a better position than doing horribly in a week. My post-bacc program had some students from the year above us come speak to us about their application year, and this one kid took the MCAT in September, submitted his AMCAS in October, and is now headed to Harvard Medical School. He probably had amazing stats, but the point is people who get their ducks in a row on the late side can and do get in--especially those who are qualified.
Another option would be to skip your test,take the MCAT next spring instead, and apply for next year's cycle. This option may or may not appeal to you depending on where you are in your life right now and whether you're already invested in applying this year.
Yet another option is to just continue working your butt off for the next week and cross your fingers. Identify your weak spots. Is it content, strategy, timing, analysis? My problem early on was timing. I used Kaplan's very specific (and I think very helpful) timing strategy to resolve this weak point. Most people get around their AAMC averages but some get below and some get above. If you proceed with taking the MCAT as scheduled, visualize yourself in that last category.
Finally, don't panic. Be calm, confident, and methodical. Figure out what you have to do and do it. Best of luck!
