In just three short lines, the speaker describes a frightening moment: seeing "the angel from my nightmare" in waking life. Something that belonged to sleep, to the unreal, has stepped into the daytime world. The word "Today" makes it feel sudden and immediate, as if the speaker is still shaken by what happened.
The last line is the heart of it: "I have lost myself." The speaker doesn't say what the angel did or looked like, and that silence is part of the effect. Angels usually suggest comfort or protection, but this one comes from a nightmare, so it seems more unsettling than holy. Judging by the title, "Loss," the encounter may have cost the speaker their sense of who they are, though the poem never explains why.
The mood is stark, anxious, and hollow. With so few words, the poem leaves a lot of empty space, which mirrors the emptiness the speaker feels.