I feel for you. I find myself sinking and sinking lately.

To prevent it, I've done the following:
*I've been trying to limit my sleep (when I sleep 14+ hours, it seems to plunge me further).
*I try to walk everyday or do some kind of physical activity.
*Journaling helps.
*Remembering it's a cyclical temporary thing.
*Being honest with others around me about it. Helps me feel a bit safer if they know. Especially with the suicidal intrusive thoughts.