Regarding relapse, here’s some interesting information from “Psychology Today“:

Contrary to conventional wisdom, it’s not some singular upsetting event that sends most people spiraling downward into a relapse of depression. Rather, it’s longstanding difficulties of a relatively severe nature.
So conclude researchers at the University of Calgary in Canada, who for six months followed 57 women who had recently recovered from a depressive episode. Within that time 28 percent of the women experienced a relapse of the disorder. The average time to relapse was 111 days.

The study was undertaken to shed more light on the process of relapse, as depression is a highly recurrent disorder. Up to 60 percent of those who recover from a single episode of unipolar depression can expect to have at least one additional episode in their lifetime.
Life stressors that are ongoing, lasting for months or even years, can wear down coping skills, the researchers found. However, the existence of social support can prevent relapse, even in the face of enduring difficulties.
It’s not that severe life events don’t have an impact, but their effect depends on the way people cope with them. Those who call on task-oriented coping skills do best.