barfer, don't assume that because quant is easy for you, that it's easy for everyone. Everyone has his own challenges, weaknesses, strengths and abilities.
Jovanne, I do believe you are needlessly stressing yourself out. It's very common for your scores to fluctuate as you prep. Take a break from timed tests and work on untimed practice questions with the Official Guide.
Also, be aware that
Manhattan GMAT's tests are harder than the real test, esp for quant. They themselves admit that some of their quant questions involve too many steps to solve, more than we reasonably have time to do in the time allotted. So focus on official questions, take the GMATPrep tests, and maybe add the Princeton Review tests. They're much more accurate than
MGMAT in my opinion.