The speaker of this short poem is admitting something they never saw coming: they have become a drunkard. The title, "Confessional," sets it up as an admission, and the first lines have the flat, stunned tone of someone saying it out loud for the first time.
The middle line is the strangest. Shame is called a "white horse," an image that usually suggests a rescuer or something noble. Here it seems twisted into something the speaker rides or is carried by, which suggests shame is driving them rather than helping. Then comes the sharpest line: "I fear myself more." The danger is no longer drink alone but the person drinking has become.
The ending turns to hiding "from the night and foul weather / of my metamorphoses." Change is pictured as a storm the speaker shelters from. The mood is quiet, anxious, and self-wary, with very little drama in the words, which makes the fear feel more real.