The only resource you mention that actually teaches you how to get better on the test is the SC strategy guide. Doing OG questions or taking practice tests is a way to practice / test yourself on whether you have gotten better with whatever you've been trying to study or learn - but simply doing a bunch of problems is NOT how you get (much) better. (As you're finding out from your practice tests.)
So, yes, you need to change your study process.
Read this:
http://www.manhattangmat.com/blog/index ... -the-gmat/
Pay particular attention to the sections on time management and HOW to study. Let us know if you have any questions. And good luck! :)
Also, if you'd like more specific advice, then we need more detailed data on your strengths and weaknesses in order to advise you. Use the below to analyze your most recent MGMAT CAT(s):
http://www.manhattangmat.com/blog/index ... ice-tests/
Then come back here and tell us the results of your analysis and what you think you should do based on that analysis. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
We'll tell you whether we agree and point you towards some resources to help. You are also, though, going to need to acquire some resources that help you get better on the test (similar to our SC strategy guide). That's almost certainly going to mean spending money - there aren't many resources that are both free and very high-quality - so you're going to need to do some research to identify what you think will be the best resources for you.