What are the literary devices used in "If I could stop one heart from breaking" by Emily Dickinson?

This short poem by Emily Dickinson comprises seven lines and has an ABABCBB rhyme scheme. The meter of the poem is also irregular, with the first line having a pattern of stresses equivalent to iambic tetrameter, while the next line has only three stressed syllables. The third line also has four beats, but we then see a return to shorter lines with only three key stresses.
In the first line of the poem, Dickens uses the idiomatic metaphor of a broken heart. This is a phrase very commonly used in English, but it is nevertheless an example of figurative language: the speaker cannot stop a heart from being literally damaged but may succeed in preventing the heart from being metaphorically broken by grief. The speaker is very dedicated to her task of keeping these hearts figuratively intact: her statement that she "shall not live in vain" seems almost a vow, as she uses the imperative "shall" rather than the softer "will." This seems a careful choice of formal diction, lending gravitas to the vow.
The speaker does not specify whose "aching" or "pain" she is most dedicated to preventing. On the contrary, her desire to "ease" is universal; if "one pain" is cooled, she will have made a valuable contribution in life. The image of the "fainting robin" being restored "unto his nest" is another metaphor: the robin is not meant literally, but serves to represent even the smallest or most seemingly insignificant person who may need assistance, which the speaker is happy to provide.
Parallelism in the structure of the poem serves to emphasize the fact that only one "aching" or "pain" or "fainting robin" needs to be helped in order for the speaker to feel useful. Repetition of the statement, "I shall not live in vain," provides a sort of envelope for the center of the poem, encapsulating the details of the speaker's vow. The speaker dedicates herself to figuratively "cool[ing]" whatever pain she finds in the world.