If you're exhausted and bored out of your skull, don't study more. Its unfortunately that you're stuck in such a busy job, but you're not the only one--and this board is filled with people who've beaten the GMAT on limited time frames.

20 hours a week is a LOT of studying. Don't force yourself to do more, because if you're burned out and tired for the GMAT, you'll bomb it no matter how well you prep. What you do want to make sure you're doing is spending that time as efficiently as possible. Are you focusing on your weak points? Are you letting go subjects tested only rarely to focus on high-point topics? Are you reviewing old material regularly so you don't need to re-teach yourself material you've forgotten?

One suggestion I have is working for a few hours with a Kaplan Tutor--since you've already completed a Kaplan GMAT course, you'll qualify for a discounted rate. Even a few hours with an expert can go a long way towards helping you realize your weaknesses and solidify a good study schedule. This in turn can go a long way towards alleviating the boredom and the stress.

Whatever you decide, I wish you the best of luck on Test Day!