This sentence could be an expression in a novel or poem. It could be understood as the following: Goodbye means the end of the story. The character will leave this world and never come back. This kind of expression was often used in farewell or parting scenes. The phrase "grow old alone" in this sentence means that a person will eventually spend the rest of his life alone and no longer share his life with others. "Lost you" is a metaphor for a person's actions or the influence of things to bring someone or something out of their original position or environment, causing them to lose contact or position. This sentence could be an expression of sadness, reluctance, or helplessness, implying that the character was about to leave this world or had experienced some difficult life experiences and finally chose to face the future alone.