Keep an
error log and go over it religously. For example, if you get # 2, 10, and 12 wrong today, do those same numbers tomorrow and the following day...and so on until you no longer get them wrong. In addition, for those numbers, work on the basic concepts of each; first start on easy problems, then medium problems, and then finally hard problems, etc. I use an Access database, located here
https://www.gmatclub.com/phpbb/viewtopic ... ght=access, to identify the level of each problem in the OG 10th edition.
Good luck and don't give up!