I think this song is about the pain we can cause others without realizing it. From a moment of anger or a moment of inattention that can destroy everything they have built in that relationship until then. And the pain he feels is the regret that he did something that should not be done, and because of that he lost everything. In the video, the woman he is talking about is filmed along with some butterflies that symbolize death. So the man did a verrryyyyy massiiiivvveeee mistakeeee upsies