the lower brain is sometimes called the lizzard brain, reptillian brian or primitive brain because it controlls instinctual survival behaviors that are less sophisticated. the limbic system and amygdala control primitive responses to stress / fear. these sections of the lower brain are concerned with very short term outcomes because, for the vast majority of human existance, threats required an immediate and forceful response (fight or run). this response isn't necessary as often these days, but the software is running in overdrive by responding to preceived threats like social anxiety, financial stress, relationship drama, work pressures, overtraining, etc.
upper sections of the brain, which evolved later, controll complex problem solving, introspective thinking, emotions, language, learning, memory, empathy and intrapersonal skills. this is where intentional thought, reason and informed action happens. this is where concious decisions are made. the upper brain thinks before doing and considers long term outcomes.
the mind body instinctively priotitizes short term threats over long term priorities and health. when the mind-body is stressed, the lower brain is prioritized to respond to a preceived threat (even if there is no real threat). since stress response is designed for immediate action, connection to the upper brain is choked which weakens emotional control, language, problem solving, perspective and most processes that would help to understand a situation. your lower brain doesn't care about unerstanding a situation, it just needs to do something about it now. this is fight or flight.
this is why perspective is difficult when you're dealing with a stressful situation. your stressed brain wants to react and get through whatever is happening. that's been its' MO for most of human existance. it's difficult to weigh pros and cons, to consider multiple angles of a problem or to think objectively when the pressure is on, because connection to the upper brain is chocked by fight or flight.
as stress accumulates over time, nervous system tension creeps up. as this tension rises, your bandwidth or adaptive capacity declines. if nothing is done to bring that tension consistantly back to balance, your daily operating tension will rise to become sympathetic dominant on a regular basis.