Four short lines, and a lot of darkness packed into them. The speaker says that while drinking, they almost kill themselves. The wording is blunt and offered without explanation, which makes it feel raw, as if the admission slipped out. We can't tell whether the danger is from the drink itself or from the despair behind it, and the poem leaves that open.
The last two lines turn to an image. A "promised joy" has been "buried like a match in the night." A match is small and bright, a brief hope of light, and burying it means it never gets to burn. The title, "Guttering," points the same way, since a guttering flame is one flickering close to going out. The speaker seems to feel that some happiness once seemed possible and has been lost or smothered. The mood is bleak, quiet and fragile, with just a faint sense of how close the speaker came to the edge.