Hi,
Thanks for your post. It sounds as though you have a very demanding job and stressful situation. I understand your frustration with your GMAT score given your desire to attend a top MBA program. However, I strongly encourage you not to quit your job. The adcom may have concerns about your lack of employment and this could impact their opinion of your overall application. They may be concerned about your ability to balance work/life as most people work full time while preparing for the GMAT. These concerns may cause them to worry about your ability to balance your school load and your extracurricular activities while at school. Secondly, there is some extra pressure to put together a solid application when you are aren't working full time when you apply. The thought is that if this is all your doing than it should be perfect. Lastly, anytime there is a gap of 3 months or more in employment you are required to explain this gap, my concern is how would you explain it?
While I really do understand your situation and I do fee for you, I fear for the repercussions quitting your job may bring. In the end, this is a personal decision and only you can decide what is best for you. I wish you luck!
Best,
Stacey