These lines come from a poem called "The Other Side of a Mirror" by Mary Elizabeth Coleridge. In it, Coleridge challenges the prevailing standards of femininity against which she and all other women in Victorian society were judged.
On this side of the mirror, the speaker is a respectable embodiment of middle-class womanhood. But as she looks deep into the mirror at her own reflection, a shocking alter ego is revealed—a woman "wild/with more than womanly despair".
Through her open lips, the woman in the mirror displays the wound of silence imposed on women in society. The absence of women's voices in society is reflected in the "parted lines of red"—those wide open lips that reveal to the speaker her true, banished self. It's as if those lips have been painted on to please men, but they do not allow the woman express herself. In a patriarchal society, she is expected to remain silent.
The speaker has been confronted with the terrifying image of herself that lurks beneath her refined, respectable exterior. Now she can see the truth of her subjection as a woman. Yet somehow she must keep this a secret; the wound of her silence must remain unexpressed. As a woman in Victorian society, she has no voice.