It’s so difficult to convey your feelings into words, but I’ll give it a crack...
Recalling a memory of sitting talking with my mother could make me sad because I know one day she will die. It could have been a great conversation, but looking back makes me feel sad.
I could be at work and a song comes on the radio that I’ve heard before and don’t associate any particular memory to it, but it makes me feel sad because it reminds of say a year in time like 2004 and the memories that come from that year. I purposely don’t listen to music so as to not trigger myself.
Sometimes the beat of a club-style song that I’ve never heard before could bring back memories of a year I went out clubbing hard.
Sometimes I wish I could just erase my memory so I can’t be taken back to those times.