How could "The Mirror" by Sylvia Plath be paraphrased?

Before one can paraphrase a poem, it's best to fully understand its meaning, and then work your way backward from there.

Let's look at the poem's most significant verses: 

I am silver and exact. I have no preconceptions.
Whatever I see I swallow immediately
Just as it is, unmisted by love or dislike.

A mirror reflects a clear, undistorted image. As the speaker refers to themselves as the mirror, they know that what they see is true and that there is no external force at play. The speaker believes he or she is clearsighted.

Nothing has caused them to doubt their identity, they have accepted all of their traits without dwelling on whether they consider them flaws or advantages, and they have no plans to adjust their identity in order to fit an idea. They are stable and self-assured, and what they see is simply the truth. This perspective is what the mirror represents. 

The poem then goes on to say:

Most of the time I meditate on the opposite wall.
It is pink, with speckles. I have looked at it so long
I think it is part of my heart. But it flickers.
Faces and darkness separate us over and over.

This is the beginning of the speaker’s identity crisis. The speaker was convinced that what they’d constantly seen—the pink, speckled wall across from it—was all that there was and that there'd ever be, i.e., a simple and unchanging life of satisfaction. 

The flickering, caused by faces and darkness, is when the speaker begins to doubt what they see. They interrupt the mirror's point of view—the only thing the speaker had come to know. Looking at the poem as a metaphor, they could be speaking respectively of the people coming in and out of their life and the development of a deep depression or a tragic event. This is only speculation, but it’s important to make connections in order to better understand a poem—especially one from the apparent point of view of an inanimate object.

Now I am a lake. A woman bends over me,
Searching my reaches for what she really is.
Then she turns to those liars, the candles or the moon.

Here, Plath describes a lake as a distorted and confusing reflection. The woman who seeks to find herself in the lake’s reflection will not see her true self, because this time there are external forces at play—moonlight and candlelight, reflected in the lake along with the woman's face. 

I see her back, and reflect it faithfully.
She rewards me with tears and an agitation of hands.

The transition from being a mirror to a lake could represent internal struggles as a person in an identity crisis, going from a routine contentedness in their own life to a constant experience of sorrow and frustration no matter how hard they try to go back to normal by "faithfully" reflecting the woman's back—who obviously ignores their efforts.

The emergence of the woman, however, can also be seen as a representation of the speaker's internal struggles. This is shown through the following lines:

I am important to her. She comes and goes.
Each morning it is her face that replaces the darkness.
In me she has drowned a young girl, and in me an old woman
Rises toward her day after day, like a terrible fish.

The speaker describes the lake as an important element in this woman's life in order to show that understanding one's identity is important. The woman returns again and again, but still she cannot truly see all of herself among the flickering moonlight and candles, and this agitates her, until eventually, in her efforts, she loses the parts of herself she knew—represented by the drowning of a young girl, as symbolism of the loss of youth and joy—and sees only the version of herself she hates—an old woman—jumping back out at her "like a terrible fish."

It's more evident now that the poem tells the story of a sequential descent into crisis, going from the emerging insecurities of the speaker (faces and darkness), to a complete and significant change in form (mirror to lake), to observing and affecting others in their search for their identity (the troubled woman). 

With this analysis at hand, it is much easier to paraphrase such a powerfully complex poem by keeping the essential elements of its theme at the forefront.

For example: 'The Mirror' by Sylvia Plath is a metaphorical expression of an identity crisis, using the clear and undisturbed reflection of a mirror and the chaotic, deceitful reflection of a lake to symbolize a shift from a stable sense of self to a sense of self muddled by external forces. This is even further represented through the emergence of a woman in the poem who cannot seem to find a whole and truthful image of herself in the lake's reflection, thus emphasizing the speaker's identity crisis through their uselessness as a source of reflection and providing a manifestation of the speaker's feelings through the woman's struggles.