Hi Lily, here are my suggestions for you...
First, now all the O.G. problems like the back of your hand. Do them all, then do them all again.
Second, do all the PR problems you can get your hands on. These on average aren't as hard as some of the real test questions, but the strategies you develop apply to the GMAT questions.
Third, do the challenges on GMATClub. These are very valuable!
Fourth, do the Kaplan Math workbook stuff. These tend to be very difficult, but they will solidify your skills.
Good luck!