Hi ConfusedParrot,
Based on what you describe, there could be a number of factors that are causing you to get tired. Do you get this tired when you work on Quant questions? What type of environment are you studying in and what time of day are you typically studying (re: are you studying more often at the end of the day)?
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich