"Plath’s great achievement was her ability to transform the experience into art without losing its nightmarish immediacy" (Critical Survey of Poetry).
This quotation sums up Plath's style of poetry which was to use her own experiences to create her art without coating the bad parts with any kind of sugary gloss. Such is Confessional Poetry, a subgenre of poetry that draws upon the autobiographical experiences of the poet. Plath's poetry and her novel Bell Jar are excellent examples of this style of poetry.
Even though Plath's father died when she was eight, her fear of and revulsion for his deeds are vivid and jarring in the poem "Daddy." In this poem, Plath makes references to her father as a "devil," a "black man" and "bastard." This pain comes from his association with the Nazi party and the conflict she feels because of the Jewish heritage she has from her mother's family.
Her pain after her divorce from Ted Hughes is evident in the poem's intense imagery. These poems and many others use the first person pronouns to make it more clear that Plath herself is the speaker. This makes the confession aspect of the poetry easier to see.