I agree that a prep course might be a good idea. Sometimes there is a ceiling that students hit when studying on their own.
Now, you said that you "did" the problems. The word "did" means many different things, depending on who you ask. Here's how to tell if you truly "did" a problem:
1. You know the category of the question (e.g., "Yes/No Data Sufficiency with Algebra and Plugging In")
2. You discovered the trick/strategy to attack the problem - note that this may not be the same way that the OG suggests you solve the problem
3. You know why every answer choice is wrong (for verbal) or why certain answer choices were included as traps, partial answers, etc. (for math)
4. You could do the question again tomorrow, without thinking about it, and get it right in less than a minute
5. You could change the numbers a bit (for math) and still get the question right
6. You could make up a similar question on your own
7. You could solve a similar question if presented later
8. You could explain to someone else how to solve the question, why your way of doing it is best, and why every answer choice is right or wrong.
Most people don't study this intensely, but it's the key to upping your score into the high 600s and 700s. That last point is one of my favorites - I felt like I reached GMAT enlightenment only after I started teaching, and truly had to break down every problem so that I could explain them to students. Pretend that you have to teach a prep class, and you're going to be expected to know how to do explain every problem in the OG, and preferably have some sort of trick or "lesson learned" from every problem.... that should keep you busy prepping for awhile!
Jim S. | GMAT Instructor | Veritas Prep
