I don't have a ton of experience with the GMAT, but I took it recently and did fairly well (710 Q49 V38). I scored 620 (August) and 680 (October) on two practice exams. I don't recommend only taking two, and I surely don't recommend taking them that far apart.
It sounds like you felt pretty good about much of the test with the exception of the reading comprehension and critical reasoning questions. In the weeks before I took the test I wasn't completely confident in these section either. What I did to get better was a combination of working through as many example problems as I could and reading things having nothing to do with the GMAT.
Reading in general helped with my confidence in being able to read a passage and have some sort of semblance of what the author is trying to get across. The one thing I really focused on while reading was being as active a reader as possible. Why is the author telling me this? What is the purpose of the passage? Why is the passage organized this way? And so on.
While working through example questions I knew that being an active reader was the only way to be able to efficiently answer the succeeding questions. I took a little more time to better read the passage and answer questions in my head, then when working through the questions I generally didn't have to do a whole lot of rereading any portion of the passage.
The questions that appear to have been difficult for you are questions about inference and purpose. This is understandable given most people's instinct is to read for content rather than content and motivation. Again this is where actively reading helps.
What I've written above is mostly focused on reading comprehension, but it is 100% applicable to critical reasoning. Ask yourself the same questions and take it a step further and ask yourself: What are the flaws in their reasoning? What would hurt or strengthen the argument? Being able to think of questions while reading will help a great deal.
Lastly, at one point I did some reading about the purpose of the GMAT. The one thing that sticks with me is that the whole purpose of the GMAT is to see what you are capable of given limited information. With this in mind, you have no choice but to actively read and think through every question, not necessarily trying to answer the question while you read it, rather work through reasons certain pieces of information are given and other pieces of information omitted. This helped me a great deal.
I hope I helped a little bit. If you have any questions let me know!
AustinLM