On occasion some of us are exposed to frightening and painful events way out of the ordinary that can leave us with mental scars. These may well heal over such that, at a superficial level, life goes on largely unchanged. Underneath however the effects can last for years, even decades.
These can manifest as a peculiar detachment from the world arounds us or a low level depression which goes on and on. At the same time it can result in eruptions of anxiety precipitated by cues which bring the past trauma to mind. We may find ourselves, even unbeknownst to us, avoiding these. We might avoid them during the day only to be plagues by them at night as nightmares.
Post Traumatic Stress Disorder (PTSD) is a distinct condition which responds best to a it’s own particular types of medication and psychological treatments.