The speaker is with one person but longing for another. Every line repeats the same pattern: a wish for "you" and a rejection of "him." The details are intimate and physical, like smelling someone in her hair or undressing, and each time the speaker wishes a different person were there. She says she is "dying" to hold the one she misses and would "give anything" to be with them again, so the feeling is intense and a little desperate.
The repetition makes the poem feel like a confession she can't stop making, or a chant she tells herself in private. Then the last line changes everything: "I'm such a good liar, though." It's unclear what she is lying about. She may be hiding her true feelings from the people around her, or the whole confession could be a lie she tells herself. Either way, the mood is sad and unsettled, and it leaves the reader doubting what to believe.