Some schools will accept the January MCAT, but others will review your file as soon as your MCAT is in and will make a decision at that time (assuming you submit your application prior to getting your scores).
You should just take the MCAT when you're ready. We all have second thoughts right before we take the exam, but if your practice scores aren't what you are looking to get on the real deal then push it back, because the odds that your score on the real exam will increase are very slim. You will likely be right in range with your practice score averages.
I'm sure you're just experiencing cold feet. It's natural, but you have to just remember that at the end of the day it's just an exam. Just relax and nail that beast. Good luck on your exam.