The speaker describes themselves as a changeling, a figure from folklore: a creature left in place of a stolen human child. From that image, the poem builds a quiet, bitter sense of not belonging in one's own life. Even the birthday is dismissed as "water in wine," something diluted and accidental, an "impertinent chance" the speaker refuses to care about.
The mood is dry, prickly and a little defiant. Instead of asking for comfort, the speaker says "Give me rage / and I shall go on," as if anger is the only fuel that works. The last lines are the sharpest. The "man in whose place I troll" was "born happy," which suggests that the real, original self was content, and the speaker is a stand-in who took over that life. The word "troll" is interesting: it hints at a monstrous creature, but also at someone lurking and provoking. The feeling underneath is alienation, mixed with envy of a happiness that was never really theirs.