I have a friend who's an Iraq veteran and has TBI and he has intermittent memory problems. Some types of knowledge seem to be more difficult for him to learn/memorize now than they used to be. He says it's so confusing because it's not consistent. I think he may get test anxiety, too, although he hasn't told me that. (Who wouldn't, if you didn't know whether or not your brain was going to cooperate that day?)

Maybe you can ask your instructor or someone at the VA for tutoring for people with TBI?