Most of the lines in “Sonnet 30” are end-stopped. (Indeed, the poem has only two enjambments, in lines 1 and 10.) This gives the poem a slow pace and a meditative feeling. Pausing at the end of most lines, the speaker feels exhausted, overwhelmed by despair and regret, unable to keep going.
That feeling is especially strong in lines 5-8:
Then can I drown an eye, unus'd to flow,
For precious friends hid in death's dateless night,
And weep afresh love's long since cancell'd woe,
And moan th' expense of many a vanish'd sight;
Here the speaker describes a series of sorrows—the death of “precious friends,” lost loves, etc. Each line introduces a new grief or disappointment. And each line is end-stopped. The end-stops slow these lines down: each line is pensive and heavy. As a result, it almost feels like the speaker is hesitating at the end of each line—delaying the next line, avoiding discussing the next form of grief or sorrow.
End-stop generally works this way in the poem: slowing it down, emphasizing the speaker’s sorrow and regret. In the final couplet of the poem, however it plays a different function. Both of the final two lines are end-stopped:
But if the while I think on thee, dear friend,
All losses are restor’d, and sorrows end.
Here the end-stops—particularly in the final line of the poem—communicate certainty, confidence, and finality. The speaker has no doubts about the power of love, its capacity to restore losses and end sorrows. Although these lines are end-stopped, the end-stops function differently than they do in the rest of the poem: no longer expressing sorrow and regret, they embody the speaker's confidence in the power of love.
Line 13, however, could also be considered an instance of enjambment, since the meaning of the phrase ("if the while I think on thee") isn't completed until the next line. However, the incompleteness doesn't convey uncertainty. Rather, it suggests excitement. Again, this contributes to the overall confidence in love that the final couplet talks about.