Hi,
If you are feeling skeptical about how you are going to do on the exam, it may be best to postpone it until mid October and go for the R2 applications. It would be better for you to go into the exam more comfortable and with better preparation, rather than having to retake just a month later to meet the deadlines. With that being said, I would prepare ahead of time, and put in the time researching now what the qualifications recommended are for the schools you want to apply to. This is beneficial, because if you get a score you want, you will already be prepared to start getting everything out to the schools you want. Have you been using an adaptive course? Is this your first time taking the GMAT? If you need any study tips or have any other specific questions, please feel free to ask.
Good luck and all the best with your studies!
-Evan