To come up with lyrics to an already finished piece of music, I'd probably start by taking note of how I feel as I listen. Perhaps that would lead to writing down those feelings or impressions and that in turn might lead to more specific ideas and eventually to the point of the song or the message I want to convey. Brainstorming, I guess.

Dark and gloomy -> trouble -> what kind of trouble -> relationship, money, car trouble....

If you think about it, you probably already have some ideas for what the song 'could' be about. Write them down. See if any of them appeal to you. Ask yourself why? Write down the answers.

Is there a title? If so, why did you choose it?

Good luck:)

My songs are here:
https://soundcloud.com/scott-h-olson