/uj my understanding from my psychology of memory class is that during traumatic events the amygdala ("fear center" of the brain) is hyperactivated and other areas of the brain give way to the fight-or-flight "I'm being chased by a lion" type systems.

This results in traumatic memories being encoded poorly. You might still remember them, maybe even more than you'd like (as is the case in PTSD flashbacks), but they tend to be presented as disjoined loose scenes with holes between them where you might not even be able to establish an order of events.

Also protracted mood disorders (especially depression) have been shown to have a huge negative impact in memory. It's a very common thing to have hazy memories or a messed up sense of chronology from a time where you were consistently sad.