Answer:
The answer is "Don't cut your fingers with the edge / of your keen wit."
Step-by-step explanation:
After a thorough reading of the poem, we may conclude that the speaker is advising Helen Grey.
The speaker tells her to be careful. Helen is too confident due to her beauty and the attention she gets from men for it. She seems to have forgotten that beauty fades away as the years go by. And, with her beauty gone, the love and attention she is used to receiving will also disappear.
The speaker believes Helen will end up hurt if she does not become humbler:
Don't strain the cord until it snaps,
Don't split the sound heart with your wedge,
Don't cut your fingers with the edge
Of your keen wit; you may, perhaps.
Because you're handsome, Helen Grey,
Is that a reason to be proud?