Astrophil laments how he has lost and wasted the gifts that Nature provided him with at birth. Heaven has "lent" him such qualities as his "youth," "knowledge," and "wit." He describes how Reason judges him to be lacking in his actual use of them. Astrophil agrees, finding himself "bankrupt" and unable to pay "Nature's rent," that is, to employ the qualities Nature has given him to good ends. The first twelve lines thoroughly indict Astrophil for not using the gifts of Nature and Heaven well. In line twelve, he describes how he sees that he is about to "lose [him]self," leaving his good qualities behind entirely.
As usual with English sonnets, the final couplet presents a turn to the unexpected, something that changes the reader's perspective on the poem as a whole. Throughout the poem, Astrophil has been chiding himself for his failures, but he never explains why he fails. (In the context of the sonnet sequence, it's pretty clear that it will have to do with Stella, but since the sonnets were circulated individually before being published as a group, the contemporary reader might not have had that context.) In the final two lines, however, not only does he reveal that Stella is the cause, but he also transforms the emotional context, praising what he has been lamenting: "I see and yet no greater sorrow take / Than that I lose no more for Stella's sake."
This rhetorical move is similar to the American Nathan Hale's later "I only regret that I have but one life to lose for my country." Hale was saying that although he valued his life, he valued his country so much more that he regretted how limited his sacrifice could be. Similarly, Astrophil does prize his natural qualities of wit and knowledge—he spends most of the poem telling us how much—but he prizes Stella so much more that he would be willing to give anything he had for her. He's sad, therefore, that he doesn't have more to give. As with many sonnets, the entire poem sets the reader up for the impact of the final couplet.