Well, the story behind it might vary depending on who interprets it. But generally, it seems to convey the intense pain and emptiness after a significant relationship ends.
The amount you can quote lyrics in a story varies. Generally, fair use allows for a limited portion, but it's best to seek permission if it's a significant amount. Also, consider the purpose and context of the quote within the story.
No, it is not appropriate. Such topics are vulgar and inappropriate for public discussion. We should focus on positive, healthy and respectful relationships in communication.
Sure. How about 'gf and bf compare their dreams and support each other?' It focuses on the positive aspect of a relationship where they share and encourage each other's aspirations.