The speaker feels empty without someone else. They describe themselves as a "dark moon" full of sorrow, and the person they love as a bright sun. The image works neatly: the moon has no light of its own and only shines by reflecting the sun. In the same way, the speaker believes they get all their light and strength from this other person.
The feeling here is need, close to desperation. The speaker doesn't hide behind clever language. They simply say that everything in them depends on this "bright light," then plead, "Please do not leave me." The last line asks for even more of that "everlasting" light, which suggests the speaker wants a love that never fades or runs out. The title, "Fallen angel," and the subtitle "Night" add to the picture of someone cut off from brightness and waiting in the dark. The mood is vulnerable, sad and hopeful at once, like a quiet prayer to the one person who can make things feel bright again.