Whatever I see, I swallow immediately. Just as it is, unmisted by love or dislike (2-3)
These two lines show two things: 1) the transformation of a mirror into the speaker through personification, and 2) the mirror's resistance to transformations of any kind in what it reflects. The transformation of the mirror here is shown by its proclaimed ability to see and swallow. The poem has transformed an inanimate object into something with the ability to function as if it has eyes and a mouth.
I have looked at it so long I think it is a part of my heart (7-8)
These lines show the transformation of the pink wall into something more meaningful to the mirror. This transformation, unlike the transformation of the mirror into a lake, or of the woman from young girl to old woman, is not physical but emotional. Ironic, considering neither mirrors nor walls have emotions.
Now I am a lake (10)
The transformation from mirror into lake is abrupt. In the second stanza, we'll see the lake, who takes the same pride in its exact reflections as the mirror does, be useful in showing the drowning of youth and rising of old age more vividly than the mirror could. After all, it wouldn't be as thorough of a metaphor if the young girl drowned in her bathroom mirror. And it'd be pretty laughable if a terrible fish started rising with the background of a pink speckled wall.
In me she has drowned a young girl, and in me an old woman Rises toward her day after day, like a terrible fish (17-18)
These lines show the importance of appearances to the woman as time passes, transforming her. The transformation in these two lines, from young girl to old woman, is the part of the poem that all women – and all people (men don't stay little boys forever) – can expect. If they're like the woman in this poem, they'll anticipate it with fear.