When something stressful happens, the brain prepares the body for action. The hypothalamus, pituitary gland, amygdala, locus ceruleus and opioid system all release hormones to speed up respiration, raise blood pressure, reduce sensitivity to pain, all useful conditions for the proverbial fight or flight.
Under normal stressors, as soon as these hormones are released, feedback systems go into operation. The hypothalamus tells everybody else that their job is done and they can back off.
These hormones, especially cortisol, damage brain structures, notably the hippocampus, whose job is to regulate emotion and to perform the "that was then, this is now" function. I named it that, and am very proud of it. My own brain has almost no "that was then, this is now" function. Pretty much zip.