One of the most important companions for me when I'm in a funk is to remind myself that it won't last forever. It's a temporary condition, a break, so to say, from our more preferred mood states. When I do this, I'm often able to learn from the experience. The reminder is now automatic for me, and helps me to sit with my sadness.
It also helps to remind myself that people care, even when I'm not in contact with them during those darker moments.