What I've learned is that the authentic happiness I felt while married and raising a family - now replaced by singledom and an empty nest - will take great effort to achieve through activities that offer temporary relief and only a semblance of the real thing.

I wonder why the loss of happiness feels like a major internal organ is gone, leaving a huge empty space inside?

This sounds like serious depression to me, not just reduced happiness; you might consider method 1 first. Sympathies and best wishes.