Hi merajul,
I think that pushing back your Official GMAT would be a good idea (whether that future date is sometime in September or some other time is up to you).
As far as your ongoing studies are concerned, since you work such long shifts, you might be too tired after a day of work to study in an effective fashion. As such, you might find it beneficial to adjust your routine a bit. If you can go to bed earlier and walk up earlier, then you can study BEFORE you go to work - when you will likely be more awake, alert and able to learn new GMAT concepts.
Given your score goal, you would likely benefit from investing in a GMAT Course of some type (either Guided Self-Study or instructor-led), Most GMAT Companies offer some type of free materials (practice problems, Trial Accounts, videos, etc.) that you can use to 'test out' a product before you buy it. We have a variety of those resources at our website (
www.empowergmat.com). I suggest that you take advantage of all of them then choose the one that best matches your personality, timeline and budget.
If you have any additional questions, then just let me know.
GMAT assassins aren't born, they're made,
Rich