Preparing for the MCAT is like preparing for a marathon. You cannot run 1 or 2 miles a day for two months and then expect to be successful in a 26.2-mile marathon. Runners vary their running time and build up to the 26.2 miles, routinely running long distances in preparation for marathon day.
How often are you studying and for how long? If you are only studying in 30- or 60-minute increments, it could be that when you sit down to complete a Full-Length Exam that you have not built up your endurance for that type of scenario. Not only do you need to study and practice regularly, but you also need to simulate the Test Day experience of concentrating for 6 hours and 15 minutes of testing time. Gear up for practice tests—and Test Day itself—by periodically holding long study and practice sessions. When working on CARS, try studying in 90-minute increments to mimic the amount of time you’ll have for this section.