They are inseparable. Specifically, when you’ve had a lot of stress like occurs in PTSD or in child abuse, you find an upregulation of kappa opioid receptors (over-sensitivity to the chemical) in the amygdala (fear/stress center) and often things like increased startle response etc etc… it all makes perfect sense.

Basically, your biology is designed to record bad events and respond accordingly in the future. The older you are the more bad events are recorded and therefore the more patterns of danger one can pick out.

This also means more false positives in pattern recognition and if the direct fear linkage isn’t removed basically the world is generally seen as a bad place with danger around every corner.

The point is to remove the connection between cognitive memory stored elsewhere in the brain and the fear linkage in the amygdala which is often mediated through kappa opioids.

There are other complex systems that make good design sense as well but you can get lost tracking down neurochemical cascades so I’ll avoid the digression.

The short answer is that physiological and psychological are just two views of one process.