Remember that 630 - 650 puts you well above most GMAT test takers. Knowing the fundamentals will get you to that level, but please do consider that you are competing against all of those other people who likely also know the fundamentals. What you need to do is to make sure that you learn from each mistake you make and continue to improve.
That is my own personal standard. I know that someone who knows the GMAT as well as I do should never miss a question, but believe me, we all do. Missing a question is not what concerns me, what concerns me is did I learn from it. If I missed something in say, geometry, and if I learned that you can apply a particular rule to determine a seemingly impossible-to-find angle then I am excited because this is something new and missing that question was well worth it to learn. It does not have to be the case that I did not understand the underlying formula or equation it can be enough that I learned that it can be applied in a new situation.
The things that you want to try to eliminate are those mistakes that you should not have made. If learning new things is another step on your GMAT journey, missing questions for the same silly reasons is like being on a treadmill. You think that you are walking somewhere but you are actually just treading the same ground over and over again. MANY people are stuck on this treadmill. If you want to improve you need to "learn" the GMAT just as you have learned the fundamentals.
You need to make sure that you eliminate all of the silly errors that create mistakes that you cannot learn something new from...then you need to focus on improving with every error. You need to look forward to making mistakes since you learn more from these than you do from correct answers.
You have less than 3 weeks so you need to be very active here. Analyze the common threads between the mistakes that you make. Are there changes that will get you multiple additional questions correct? How about your test strategy that Brent mentions? Are you finishing the sections? Are you rushing into errors?
If you provide some more information you can get some more specific advice on how to improve specific aspects over the next 3 weeks.
