My test is in under 3 weeks and I'm halfway through my first round of destroyers...
For whatever reason, the GC is far and away the hardest for me. I'm hitting 80% correct on Bio my first time through, 60% on OC, and around 20% on GC. The GC is so hard for me, it makes me forget what I know while I'm doing it.
It is discouraging. I've got another week of destroyers all freaking day, and it's hard. It exposes me to tons of holes in my understanding, and forces me to confront my inadequacy. But, that's what I need. I KNOW these questions are beyond what the DAT calls for (in GC/OC), but I also know that being able to do them will be good for me. So I will suffer on. I will do another few days of destroyers, then review, then do another round, all the while shoring up what I don't understand.
Destroyer is meant to destroy your confidence, BUT once you can do it it should have the opposite effect. It's really good to know what you don't know.