I really feel your pain. I just went a year without adding any new pages to any of my sites. Just a couple of days ago I got back into it, added a few new pages and plan to add several more each week. I think it's a variety of burn-out and you just sometimes have to take a break and come up for air.
One thing I've decided is to just ignore what any SERPs do, not just Google's, and just do my personal best with my sites. Let the chips fall where they may. I have other pieces of my life that sometimes need more attention. And I have a need to have fun whenever I can.
Set yourself a personal goal, even if it's as little as one page a month, and find time to enjoy yourself. We may not be kids anymore but we still need to play.