The best way to do bad on the MCAT is to worry, fear, and just plane talk yourself into failing the test. If you think you are prepared, then take the test. If you're not prepared, then don't take the test, its a simple yes or no.
The MCAT is more of an endurance test than something that requires large sums of brain power. The questions are generalized and cover every course that is required for med school. If its something new, then its covered in the passage, otherwise it requires basic sciences which you should have learned already to figure out the problem.
I took the MCAT in 2001, got a 24. Was not ready, dealt with supporting my family, paying for school, supporting myself and so on. Despite my score, the test wasn't that scary. The most important thing for me was to take it again when i was READY. In 2003, when life was better, i retook it again, and so I will be applying this June with a 34. I went into the test confident that i knew my stuff, gave myself ample time and focused on my weaknesses.
As the saying from Sun Tzu goes: "If you know the enemy and know yourself,
you need not fear the result of a hundred battles.
If you know yourself but not your enemy,
for every victory gained you will also suffer a defeat.
If you know neither the enemy nor yourself,
you will succumb in every battle."
Even though this is from the book "The Art of War", really if you know what the MCAT is all about, and know your own strength's and weaknesses, then you should do well. I applied this to myself, having taken the MCAT once already, I was reassured that the test wasn't as bad as it was before. Additionally, many years of dealing with the ups and downs of my study habits, and intelligence has provided me with a way to adjust my studying skills to take advantage of my strong points and avoid exposing my weaknesses (ie: procrastination!) On top of all this, one has to get your game face on and want to destroy this test, not worry about how it could destroy you.
I hope that helps. I know that its hard for you to understand it since this will be your first and hopefully only time taking the MCAT. Just know that the MCAT is just a hyped up standardized endurance test. Remember how people hyped up OChem? Wasn't so bad after you took it right?