"I am not cruel, only truthful." This is describing the mirror. Personification is used because the mirror is given characteristics.
"Then she turns to those liars, the candles or the moon." This is a metaphor, the candles are compared to the stars. They both give light. The star and moon both represent an individual side to truth. This plays back to the theme of the poem.
"Now I am a lake. A women bends over me, searching my reaches for what she really is." Plath used imagery, creating an image of sight. I can picture what the lake sees and the sadness in the women's face.