This sonnet launches right into the heart of the action, finding its speaker in utter turmoil. The speaker offers no specifics about his situation, instead beginning a series of paradoxical statements that capture the twin joys and pains of love. In other words, he's lovesick, and he's got it bad!
Almost every line offers a new metaphor—sometimes two—that describes the speaker's tortured, conflicted state. The first line refers to the speaker feeling no "peace" despite his metaphorical warring being "done" (perhaps referring to some fight with his lover that is now technically over). The absence of war is, logically speaking, peace—but the speaker feels anything but peaceful! Whatever "war" he went through is still with him internally even if it isn't actually taking place at this very moment.
The caesura sets up this paradox, with peace on one side of the comma and war on the other:
I find no peace, || and all my war is done.
This parallel grammatical structure, which will repeat throughout the poem, is an important part of how the reader experiences the speaker's fraught mindset. Think of the speaker as being pulled in two opposite directions—towards and away from the one he desires—and how that threatens to tear him in half.
The end-stop after "done" is also rather ironic, creating a solid pause despite the fact that the speaker insists this situation is far from over. Throughout the sonnet, full stops like this will create a sense of weariness—as though composing the poem itself is a torturous act, precisely because it forces the speaker to express how he feels.