But when my wond'ring eyes and envious heart Great Bartas' sugar'd lines do but read o'er,
That "But" that starts the line is pretty key. Our speaker spent the first stanza telling us that her poetry wasn't good enough for history. Now she backs off a little—she's not just here to tell us how lame her poems are.
Here she confesses that, when she reads the poems of one of her favorite poets, she feels both amazement and envy. That writer, Guillaume du Bartas, was a French Protestant, and a big favorite with the Puritans. He was definitely the kind of guy who tackled big subjects (for more on the man and his work see the "Literary and Philosophical References" section). By slipping in an allusion to Bartas, Bradstreet's speaker gives us a sense of her taste in poetry, and also lets us know that she's well read.
We love the way Bradstreet calls his lines "sugar'd." It's such a great description of the experience of reading your favorite poems—just like eating something sweet and delicious. Mmm, poems…
Fool, I do grudge the Muses did not part 'Twixt him and me that over-fluent store.
Bradstreet's language gets a little fancy here, but stick with us and we'll break it on down. Basically what she's saying is that, when she reads Bartas, she wishes that the Muses (mythological women who inspired the creation of art) had shared with her some of his supply of talent ("that over-fluent store").
By the way, she's calling herself a fool at the start of line 6, because she feels like she shouldn't be jealous of Bartas. It sort of sounds like she's calling her reader a fool, though, which makes her sound a little like Mr. T. We would love to see him read this poem.
A Bartas can do what a Bartas will But simple I according to my skill.
Finally, she decides she has to let it go, and be happy with what she has. She won't challenge the greats, like Bartas, or be jealous of them. Instead, she'll do what she can with the skill that was given to her. Way to make the most it, speaker lady.