Answer:
Omit the last two commas.
Step-by-step explanation:
Struggling; unpublished writers are sure that publishers who seek out and publish new authors are nonexistent.
We omit the last two commas because all the sentence is the same idea and when we use commas it is because we want to express different ideas or make pauses and in this context it is not necessary. Also, I will say that in the first part after the word struggling we use a semicolon in order to make emphasis about the next elements of the idea.