Someone has left, or been cut out of the speaker's life, but they haven't really gone. The speaker is stuck in a darkness that comes from thinking about this person constantly. Everything is stated plainly: "I can't get you out of my head." The person brings "pain and fear," and the speaker wants it all to stop.
The poem's central trap is that the darkness will only lift "until we forget each other," yet the speaker doesn't want to forget. The last line, "And I don't know why," is the most honest moment. The speaker can't explain the attachment, only admit it.
The mood is heavy, tired and quietly desperate. Short, flat lines make it feel like someone talking to themselves late at night, too worn out to dress things up. The title, about throwing away supermarket flowers, hints at an attempt to let go, though the poem itself suggests that throwing things out is far easier than letting go of a person.