For Donne, the nature of love seems to be torturous. Love is hard, and it makes us suffer both while we are in and when we are falling out. It's hard to stay in love, even for a short time.
Donne describes the nature of love as fickle and harsh. He comments that he carried his heart into the room, but not back out. Love is akin to torture. Love, he says, "swallows us and never chaws" (stanza 2). In other words, it never chews us up and ends our misery.
Yet nothing can to nothing fall,
Nor any place be empty quite ;
Therefore I think my breast hath all
Those pieces still, though they be not unite ; (stanza 4)
Donne finally seems to decide that although love is hard, we can put the pieces of our heart back together and love again.