Just focus on a sad moment. For example, 'Loved dog. Lost. Heartbroken.' Simple and sad.
Think about things like loss or loneliness. Such as 'Friend left. Silence. Empty room.', it shows the sadness of losing a friend. You can also consider unfulfilled hopes, like 'Dreamed big. Failed. All gone.' which conveys the disappointment and sadness when a big dream falls apart.
You can start with a relationship. 'He left. She cried. Forever gone.' This story tells about the end of a relationship and the pain it brings. Another way is to look at the passing of time and opportunities. 'Time passed. Chances lost. Regrets remain.' It reflects on how missing opportunities can lead to a sad state of mind.