Help me yse this poem?

Some of your confusion appears to come from an imprecise use of the word 'negative.' Donne was highly educated in both literary and theological discourses and it's likely he refers to a kind of "negative theology" of love, in which the transcendent truth is accessible only through negative statements. See the linked Wikipedia article on Apophatic theology for a little more on this topic. This conceit is probably developed out of this theological concept.