The Greek poet Archilochus said "We don't rise to the level of our expectations; we fall to the level of our training." Everyone imagines how they would do everything right when the adrenaline has been dumped into their system and lives are on the line, but that just doesn’t happen. When it really counts, we don’t perform better. We perform much worse. Most of what makes Special Forces special is that they train, a lot. Then they train some more.

I've read many accounts of first time in combat. The usual experience is "my brain froze, but my training kicked in and saved my life".