Use strong verbs like 'lost' and 'die' in it.
Gift, rejected, sad. Imagine you put a lot of effort into a gift for someone. It could be a handmade item or something you carefully selected. But then that person rejects the gift. It makes you feel sad because not only is the gift not appreciated, but also your feelings behind it are ignored.
Think of a character and an action. Such as 'Boy laughed, toy broke'. Here we have a boy as the character, laughing as the first action and then the toy breaking as the second action which creates a very brief story in just four words.
Use colors. For example, dull or muted colors can set a sad mood. A grey background instead of a bright one can make the picture seem gloomy.
There were many sad online names such as: Sinking into Emotions 2. Sadness and Memories 3. Loneliness 4. Heartbreak and Pain 5. Sorrow and loss Decadence and Despair Remnant Dream·Impermanence 8 Heartbreak and Pain 9 Loneliness Sadness and Tears These online names all had deep emotional implications, and one could choose a suitable online name according to one's preferences.
Think of a relationship ending. Like 'She moved on. I'm left in the past.' It's about capturing that one moment of pain or loss. You can also consider the loss of a dream, say 'Dreams shattered. All hope seems to fade.' Here, it shows the despair that comes with the loss of something one has been hoping for.
Think of tragedies like death or separation. Then condense it. Such as 'Mother gone. Empty world now.' You need to strip away all the extra details and get to the core of the sadness in a very concise way.