We know that it is just a figure of speech to say "I can never repay you," but the speaker does seem to come up short. She can't "repay" what her husband has given her. In a way, her ability to "give back" is dead. In a poem so full of passion, this is a very strange moment.
My love is such that Rivers cannot quench, Nor ought but love from thee, give recompense (7-8)
The word "quench" makes the speaker's love seem like some kind of flame (what else would water extinguish?). Her love is like an undying fire. These kinds of metaphors are common in professions of love, but it also seems like an undying fire might pose some problem. Love can, perhaps, get out of control.
Then while we live, in love let's so persevere, That when we live no more, we may live ever (11-12)
One way to defy death, not literally but sort of, is to be remembered after you die. The poem itself is a testament to the love between the speaker and her husband. It will live forever, as long as there are people who read this poem. Of course in a way, this is totally true, because people are still reading Bradstreet's work more than 350 years after it was written.