What is the effect of the rhyme and repetition in the first six lines of Sonnet 29 by Edna St Vincent Millay?
I teach my students that the meaning of a poem is one derived (that comes from) from personal interpretation and support. So, what I offer you is my reasoning behind your question.
To truly understand the effect of rhyme and repetiton in a poem, one must look at the poem in its entirety. The poem is about a woman's love and how it has been lost because love is as changing as nature and the heart is too slow to learn something that changes so swiftly.
As for the rhyme of the poem, you cannot look at only the first six lines based upon the fact that the scheme is as follows: a,b,a,b,c,d,c,d,e,f,e,f,g,g (which shows the poem is written using the Shakespearean sonnet form with the last two lines as a rhyming couplet).
The rhyme of the poem has the effect of the typical Shakesperean sonnet:
The Shakespearean sonnet affords two additional rhyme endings (a-g, 7 in all) so that each rhyme is heard only once. This not only enlarges the range of rhyme sounds and words the poet can use, it allows the poet to combine the sonnet lines in rhetorically more complex ways. Shakespeare often gave special emphasis to the break between the second and third quatrains (equivalent to the major break between the 8 quatrain lines and the 6 tercet lines in the Italian sonnet), but he also paired and contrasted the quatrains in many other ways, creating a great range of argumentative or dramatic effects.
Shakespeare invested the couplet with special significance. It often summarizes or characterizes the musings of the three quatrains in a sardonic, detached or aphoristic voice, standing in some way aloof from the more turbulent and heartfelt outpouring of the quatrains.
As for the effect of repetition in the poem, "pity me" repeated three times in the first six lines, it shows the narrator's need for empathy from the reader. What this does is asks the reader to connect to the narrator. The narrator wants the reader to feel pity for her based upon the fact that she is unable to know true love. Her love's desire for her is gone. She has the final realization that love is only known by nature:
love is no more
than the wide blossom which the wind assails,
than the great tide that treads the shifting shore,
strewing fresh wreckage gathered in the gales,
So, in the end, the effect that the rhyme and repetition have on the poem is one that exists so as to pull the reader in so that they may empathize with the narrator and to fulfill the scheme of a Shakespearean sonnet.