Edna St. Vincent Millay: What lips my lips have kissed, and where, and why

What i understood about this poem is that Millay describes how  a person is lost without his/her other half. That person feels lonely  and feels unremembered . "Upon the glass and listen for reply/and in my heart stirs a quiet pain" (654).  This line means to me that the person awaits a voice or someone to speak to that person because that individual feels empty inside which refers to "my heart stirs a quiet pain".