Hi RahulDobriyal,
Many Test Takers spend 3 months (or more) of consistent study time before they hit their 'peak' scores; since you've been studying for just 1.5 months, you shouldn't expect to have mastered any part of this process just yet. By extension, there's a reasonable chance that you'll naturally improve as you continue to study and become more familiar with the concepts, patterns and Tactics involved in the GMAT. Since you're already working with some study materials, I suggest that you continue studying as you have been and take new FULL-LENGTH CATs (with the Essay and IR sections) at regular intervals to track your progress. You shouldn't be concerned about using OG materials too 'early'; there are plenty of quality practice materials on the market.
GMAT assassins aren't born, they're made,
Rich