Just as it is, unmisted by love or dislike (3)
This line shows that the mirror tries not to be biased when judging appearances, which, for people, is a hard thing to do. The mirror says that love and dislike don't cloud the way that it views people or objects. For most people, love and dislike definitely affect how we see the world and ourselves, as we see in the woman's distress with her own appearance later in the poem.
I have looked at it so long I think it is a part of my heart (7-8)
The mirror's connection to the opposite wall shows that when we look at something long enough, appearances can become more that just physical. We internalize what we see – in the world, and in the mirror.
Searching my reaches for what she really is (11)
Here, the woman is looking at her appearance in the water, and the depths beyond, to find something deeper in herself. As we discovered with the mirror's connection to the wall, appearances are significant to our identities. But this woman is not satisfied with what she sees; appearances aren't enough for a complete, satisfying answer to what she is searching for – herself.