Hi Shwetastar2003,
To start, any question that you do NOT answer on Test Day is 'penalized' - and the penalty is WORSE than if you had just answered the question wrong. Thus, you could have potentially lost a big chunk of points on the GMAT based on the specifics of your situation. By extension, as you continue to study, you MUST make sure to answer every question on each Test - even if you are just taking a guess.
Test Day is a rather specific 'event' - the details are specific and they matter, so you have to train as best as you can for all of them. The more realistic you can make your CATs, the more likely the score results are to be accurate. Since you never took any practice CAT Exams, you never properly defined your actual 'ability level.' There's a BIG difference between working on quizzes or 'sets' of practice problems and working on a FULL, adaptive GMAT taken in a Testing Facility. Now that you've taken the Official GMAT, you know exactly what Test Day 'involves', so you train to mimic those conditions during this next phase of your studies.
Raising a 600 to a 720+ will likely require at least another 2 months of consistent, guided study - and you'll have to make significant improvements to how you handle BOTH the Quant and Verbal sections. With a September 20th Test Date, you have less than 5 weeks of study time remaining, so you might be a limit to how much you can improve in that time. This is meant to say that you would probably benefit by pushing back your Test Date (even if it's just for a couple of weeks). Based on your Official Score, you likely know the content of the GMAT well enough, so you should be focused now on practicing Tactics and learning the patterns and little 'secrets' to the Exam.
GMAT assassins aren't born, they're made,
Rich