Cummings loves his linguistic paradoxes. That's just to say he likes playing with words in a way that flips their meaning, according to a line's syntax. So we often hear the speaker include ideas of "fear" and "want" in one line, only to negate them in the following line by including the word "no." So it is, but it isn't.
Lines 3-4: The speaker states that whatever is done by "only [him]" is also his lover's doing. So nothing is ever done by "only him." It is, but it isn't.
Lines 5-6: We get that solitary anxiety of "i fear," which is then quickly countered by "no fate" in the next line. So in other words he doesn't really fear anything since his lover is his fate.
Lines 6-7: The speaker then says, "i want," only to quickly counter that desire by including "no world" in the following line. So we're guessing he doesn't really "want" anything besides his lover—who is his world.