I'm just here, pondering my brain.

I get what triggers are, I guess. I've read about the amygdala, and I understand trauma (again, I guess).
But, why do things FEEL so scary if they aren't? Why don't the triggers go away once I have the big pow realization that these feelings are from the past, I've already survived this, this isn't based in the true of now?
How can it FEEL so scary if it's not? And how can I know it's NOT scary when it feels so scary?