Use 'lovelorn' to express deep emotional pain related to love, more than just feeling sad or upset. His lovelorn poems touched the hearts of many who had experienced unrequited love.
Pro Tip 2/3
Not Reciprocal
'Lovelorn' is specific to situations where love is not returned, not just any kind of romantic sadness. Despite her lovelorn state, she held onto the hope that one day he would notice her.
Pro Tip 3/3
Duration of feeling
The term often implies a prolonged period of suffering, not just a temporary feeling. He spent years as a lovelorn wanderer, searching for a way to forget his first love.