Most who study severe trauma agree that it stops time. Trauma time is frozen time, in which the experiences of the past never become past, but remain as alive and intrusive as the day on which they happened, maybe more so. Flashbacks, nightmares, hypervigilance, constriction: all are expressions of a past that continues to intrude upon the present.
Less frequently written about is the way in which trauma can reach back behind the traumatic event itself and devalue past good experiences, experiences of attachments met and love acknowledged, experiences that preceded, often by decades the traumatic event.
These observations about trauma are best suited to explaining adult-onset trauma. It need not be the trauma of a single incident, it could be an experience as extending over years, but I assume that before the trauma there were good experiences, and good memories. These good memories are not forgotten, but too often they become unavailable as an emotional resource to be drawn upon when times are tough.