The speaker admits to living only partly in the real world and partly in the reflection of a mirror. That reflection is "fair as sand," a striking phrase: it suggests something pale and attractive, but also loose, shifting, and easy to lose. Vanity here isn't just about liking one's own looks. It becomes a kind of splitting, where a person lives for the image more than for themselves.
The last three lines turn darker. The speaker follows "the light," which is called a "mad thing," as if beauty or attention were restless and a little out of control. Then comes the unsettling image of that light "having met my corpse." The mirror seems to show mortality along with beauty, so the face being admired is also one that will die. The mood is quiet, eerie, and self-aware. The speaker seems fascinated and uneasy at once, caught between wanting to look and knowing what the looking might reveal.