First of all, what exactly is the scheduled date of your GMAT? If it's longer than a week, I believe you can reschedule. Of course, you'll still have to shell out a fee for it, but I strongly suggest that if you are not confident in your ability to take it now, to reschedule it at a later date. You'll be losing money, yes, but if you really feel that you're not in the best state to take the exam, it might be a good idea.

Now, on to the exam. First of all, a probable reason for the progressive decrease in your scores has probably a lot do with your mental state. Like you said, you're burnt out. Couple this with a feeling of despair that comes from seeing low scores, it's understandable.

A way to remedy this would be to focus on what's not working with your study routine, and to approach your study prep from a different standpoint. Because you're expecting results, and being rushed by deadlines, you worry too much about scoring higher with each test. The problem with this is that you're just putting more unnecessary stress and pressure on yourself.

Instead, try focusing on each of the questions you got wrong on the CAT exams, establish your weaknesses based on your results, and focus on analyzing why you got the questions wrong. It'll be hard to do in a week, but if you absolutely have no other choice, there are a variety of suggested 1-week plans on the website/forums. Or perhaps this is what you've been doign along? You mentioned that you've been studying for the exam for the past 3 and half months, can you tell me your study plan?

Also, as for the light sensitivity issue, I'm not really familiar such, But have you tried sunglasses? Earplugs might also help decrease the overall distractions if you're studying at a noisy environment.

Hang in there! Definitely postpone if you can, if it's after a week, ul be charged the full 250 but you'll still be able to resch. For the rest...I agree with NL48. SO much has to do with ur mental state (which is whole lot harder than it sounds esp. if you fallen off the horse one too many times..) You absolutely have to BELIEVE, without a doubt, you are capable of achieving ur score. If u have time or need a break, u could check out some "positive thinking"/meditation books, def exercise, and stock up on sleep. As for the light issue, albeit pricey, u could invest in low-wave length lamps like OTT-Lite, 3 yrs ago about $70. They'll help rest up ur eyes till gday. Also check ur monitor, see if u can readjust the refresh rate, contrast, brightness etc. If it's any consolation, I used Kaplan first and did horrible. Now I'm using a whole cocktail of prep while resorting to taking a sabbatical. I'm a horrible test taker and have been out of school for quite some time. I'd try the two diags from mba.com. Those are known to be good benchmarks.

If you feel burned out, I would say stop studying. Odds are any more studying is going to have a detrimental effect on you, so its not worth it at this point. If you don't get the score you want, delay applying until next year and maybe look into getting a tutor or taking a course.