The very title of this poem just screams defiance and frustration at someone looking down on the speaker. 'Pity' can be seen as a word with negative connotations, rather than mere sympathy. In the act of dumping the speaker, her ex has also taken on a role of superiority in being the dumper rather than the dumpee.
In this poem, the speaker (presumably the voice of Millay) is telling the listener that just like the seasons and nature, her heart will emerge renewed with hope and strength.
This poem is rife with nature imagery, most prominent of this is the use of light and dark. The writer presents the sun and moon as sources as light, and therefore hope, in images such as "the light of day / At close of day no longer walks the sky" and "the waning of the moon" ('waning' is when the moon begins to get smaller/thinner in its monthly cycle). Light can also be seen as a source of joy, and therefore in using images of disappearing light, the writer is suggesting that the end of their relationship took the joy and hope for the future from her life. This is developed by "the ebbing tide goes out to sea"; the movement of the waves away from the shore suggests that someone has been abandoned and left behind in isolation. She suggests their love is like a blooming flower, and the death of their love is like "beauties passed away / from field and thicket", therefore becoming barren and without colour.
However, by line 9, the speaker confirms "This have I known always", thus reasserting to the listener that she is not to be pitied, as she accepts that all things go in cycles, such as life and love. The nature imagery is developed as she acknowledges the violence and damage which nature causes as well as creating beauty, such as "the wind assails" and "strewing fresh wreckage". However, these images suggest that the speaker wholly trusts that love will return to her life, just as the sun comes up again every morning.
The final couplet not only switches the rhyme scheme (which was formerly ABAB) but also the direction of the poem. After insisting for the first 12 lines that she is not to be pitied, the speaker admits, "Pity me that the heart is slow to learn". Here we see that the speaker has accepted the end of their relationship as a natural process, but feels foolish or small that she did not see it coming, and therefore was surprised and hurt.
The form of this poem is a sonnet, as it is about romantic love. However, instead of suggesting that Millay is still in love with the listener, the iambic pentameter creates a strong, regular rhythm which suggests defiance, as she is still carrying on and surviving, despite the hurt the man has caused. A "Go, girlfriend!" moment, if ever there was one. By sticking to the strict rhyme and rhythm, the speaker shows that she is in control of her emotions, thus strengthening her message that she is absolutely not to be pitied.
I hope that helps!