If you really do know the material--and by "know" I mean understand it conceptually, feel comfortable with the "why" behind each process, you can visualize in your mind where and how each process happens, or how each principle interrelates with everything else you know (vs. you have memorized or become familiar with the concepts at a cursory level)...then it sounds like you are in the exact same place many of my students have been: you aren't yet proficient with the process of breaking down MCAT questions and using your knowledge (sometimes) and critical thinking skills (most often) to accurately identify the correct answer or exclude the distractors. If you were my student I would have you work a problem for me out loud, everything you think along the way, so I can see your reasoning. Then I could give you a better idea. For example, one student recently worked a few problems with me and after a while I said to her "you aren't being CRITICAL." Critical thinking involves questioning what you read, evaluating whether or not it could be true. She was letting the distractors guide her thinking, instead of her taking the position of control and using what she knows to evaluate each option, she was letting them lead her around "oh, that sounds good, maybe that's the answer." She needed to have confidence that she was the expert. She knew enough to answer nearly every question she was missing. Once she caught this vision, she became more critical, more analytical, and dropped from 1-2 avg. missed per passage to 0.5 to 1.0/passage. Every student is different. When I did the same thing with another student I realized that he was not picking up on very many, if any, of the clues and information in the passage. These are the kinds of things I think you need to look for...
Motivation wise, I always find it encouraging that understanding is rewarded and memorization is usually not--so don't be concerned about your memory getting "foggy." I would do poorly on many of my midterms from the MCAT topics if I had to take them today, but I almost never miss a question on AAMC materials. That's because my professor was testing more details and AAMC allows me to use basic conceptual understanding and solid critical thinking to get through every question. Also, if you learn the right way (conceptual learning, spaced repetition, elaborative rehearsals, deep processing) memories for semantic understanding don't tend to get foggy very fast. It is the effort to memorize crap rotely that leads to frustration because the brain kicks those random pieces of info (with no neuronal interlacing) out in the trash about as fast as a poor premed tries to shove them in.
Lastly, from the MCAT-2015 materials I've seen, make sure the measure you are using to evaluate your performance is accurate. When prep company passages veer from AAMC style it is very often down the road of too much memorized detail or requiring esoteric/tricky details. If you're going to judge your AAMC question analysis skills, consider primarily how you do on AAMC questions.
