It's not psychological. I know because there is this one absolutely OP trick I learned from a gym coach, if you feel like you are fainting, lie down on the floor, raise your legs and put them on the wall. Apparently this makes blood flow into your brain or something, and it's magic. You get much better almost instantly. Anyway, I was also told to take sugar before giving blood, so next time I do I'm gonna try both, thanks.