Hi WinnieThePooh,
The GMAT is a consistent, predictable Test, so you CAN train to score at a high level. From what you described about your studies, you essentially 'self-taught' your way through the last couple of months (using various materials and a few strategies that you tried to implement on your own). While that approach was enough to get you to a 620, it will likely not be enough to get you to a 720. You need to learn and master specific Tactics to pick up those points - and right now, you don't need to focus on 'advanced' concepts (because those concepts do NOT represent the missing points that you're looking for).
Based on everything that you’ve described, I think that you would find the EMPOWERgmat Total Score Booster to be quite helpful. Most of our clients complete that Study Plan in well under 2 months, so the time commitment wouldn't be that bad. We have a variety of free resources on our site (
www.empowergmat.com), so you can 'test out' the Course before setting up an Account.
If you have any additional questions, then you can feel free to contact me directly.
GMAT assassins aren't born, they're made,
Rich