Hi nick32,
Many Test-takers find the GMAT to be challenging, so you are not alone. Many GMATers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so since you are planning to take the GMAT at the end of September, you have given yourself almost 4 months - which is great. Scoring a 700+ is a challenging task, but the GMAT is a consistent, predictable Exam, so you CAN train to score at a higher level.
Back in March, you took an initial practice CAT/mock. You should NOT take that same CAT/mock again though (since you will likely see some questions that you saw before). Once you have your next Score, you should post back here and we can discuss the results.
1) What Schools are you planning to apply to and when do you plan to apply?
2) Going forward, how many hours do you think you can consistently study each week?
GMAT assassins aren't born, they're made,
Rich