When my mother-in-law was first married, she was in a serious car accident that caused her to be thrown into the windshield (it was in the days before mandatory seatbelts). She told us that for years, following the removal of the glass from her face and the healing of the scars, she would periodically have a small piece of hidden glass rise to the surface of her skin. Although it didn't hurt her while it was lodged beneath her skin, the glass shard became very painful again as it moved toward the surface.
In a similar way, we all have tragedies, accidents, and times when we "hit the windshield" in life. We go through the healing process and fight through the pain that accompanies our struggle. It is easy to think all the "pieces of glass" are gone, only to have an event, a person, a holiday, or the like bring another piece to the surface, and it becomes painful all over again.
Complete healing often takes longer than we think.