Hi NewSc2,
I've definitely seen this before, and it definitely can be overcome.
As you're going through your work, the most important thing to think about is what you could have done better. That's not the same as 'what mistake did I make.' Don't just think about what grammar you got wrong--think about what were the warning signs you should have seen that that grammar was at issue! Don't just think about what information you forgot in Data Sufficiency--concentrate on whether it was data you didn't have because you didn't understand the math, or because you skimmed the introduction and forgot something.
Do keep working. The Kaplan books aren't just 'tips.' The strategies therein can help guide your studies, and really help you cut to the center of the material that the GMAT is testing. That being said, as great a resource as Kaplan Premier, Advanced, and Target 800 are, nothing beats a real teacher. Consider a live or online class if you really want to focus on mastering new strategies and content.
Whatever you decide, keep up the hard work, and best of luck!