I took the MCAT a while ago (ie more than 10 years), but here are my 2 cents. I taught a prep course on the test, and this is my guess as to what may be the issue.
The MCAT is essentially one big reading exam. When I took it, it had a reading section, 2 essays, and 2 science sections. The 2 science sections tested fairly simple concepts, but in a very verbally analytical way by way of information synthesis. They had very few fact / recall types of Q's.
Chances are that you probably know the concepts well, but may just need to improve your overall reading skills. Since you have done well at school, it may be that you learn best through instructors (ie writing, hearing, and then reading). Well, unfortunately, the MCAT does not test previous lectures!
When I taught the MCAT, I found that many people knew their stuff, but bombed the MCAT because they were not good readers. i.e. studying more science stuff may not really pay off, except that it makes you more "comfortable" with the various science passages/sections.
On the contrary, I knew people that did not do too well in the science courses, but rocked the MCAT because they were good readers.
Improving upon reading skills is not as hard as it may seem. For starters there tons of books you can get on speed reading & improving comprehension. Stay away from fancy/pricy prep courses. Everything you need can be bought much cheaper by getting the right books.
Finally, try reading more and more every day. You CAN DO IT! Trust me. I used to suck at reading, and I'm still getting better. Try and read well written periodicals.
Another secret. The LSAT passages are much harder than the MCAT verbal passages. Try and get up to speed to where you can jam on those. There are MANY MANY actual LSAT's released -- another great way to prep and improve your reading skills.
On a final note: I don't think the MCAT is fair, since it overwelmingly tests too much reading. I think they should at the very least communicate this better! Perhaps they think you need to have this skill for med school, but that is bunch of crap. My kid brother sucks at reading and graduated top of his med school class.
Oh well. Hate the game, not the playas.
I wish I had your troubles. You can read about mine. I'm basically screwed for life. I'm doin life behind bars baby.