How with fine high born words now can I sing
 Thy worth when thou art the best part of me?
 What can my own praise to my own self bring,
 And is it not my own when I praise thee?
 So for this, let me here, thou there now live,
 And our dear love thus lose the name of one,
 So that, with thou not with me, I may give
 That due to thee thou should have on thy own.
 O void! oh, what a pain you'd prove,
 If your tart time off did not give sweet leave
 To fill up all my time with thoughts of love,
 Which time and thoughts do, sweet, my good sense reave,
   And that you teach us how to make one two
   With praise here for him who does hence go.

                                -- Will the Bard

In the last six lines, Will speaks not to his love, but to the void. Thus, though Will has "thou" all through, to be clear I switch from "thou" to "you." The "and" in the next to last line seems to be part two of the if. In which case, the sense is, "It would be a pain, void, if you did not give leave to think of the one who's gone ... , and were it not that you teach how to make two from one with praise here for him who goes hence."

I think. That last line is the one I don't quite get.