This is the 13506th most frequent German word.
bitteren
The German word "bitteren" means "bitter" or "to plead/request" depending on context.
The word 'bittersüßen' is a compound word containing 'bitter', indicating a mixed sensation combining bitter and sweet.
Nach einer langen Wanderung genoss ich den bittersüßen Geschmack des Weins.
After a long hike, I enjoyed the bittersweet taste of the wine.
Here, 'bitteren' is used metaphorically to describe the unpleasant lingering effect of the story.
Die traurige Geschichte hinterließ einen bitteren Nachgeschmack.
The sad story left a bitter aftertaste.
In this usage, 'bittere' describes the profoundly unpleasant emotional state of the time experienced.