A divorce decree perhaps. That's the first bittersweet pill that I've lived through twice that came to mind reading the mixture of emotions in the poem.

A long time ago I chose this Beatles lyric to represent bittersweetness:

Words are flowing out like endless rain into a paper cup

They slither wildly as they slip away across the universe

Pools of sorrow, waves of joy are drifting through my opened mind

Possessing and caressing me

The occasion was graduating from high school.