These things happen. It's why one is recommended to do a couple of OW sessions before the race, so that you can get cool with it.
What annoys me is that I've done all that before, but not enough or recently-enough, and clearly for me it doesn't last. Kinda like inoculation against vertigo by going up things, one abseil in 1996 was enough for 5 years, not 10. So when it all went wrong, I was in a wave of panic and none of the tricks (lay on your back and scull, breathe, get out and back in, loosen the torso of the suit, hang back from the washing-machine) were on my horizon: only getoutgetoutGETOUT.
Bah. Kicking self. Unfinished Business for sure.