And you would accept the seasons of your heart,
even as you have always accepted the seasons that
pass over your fields.
And you would watch with serenity through the
winters of your grief.
Much of your pain is self-chosen.
It is the bitter potion by which the physician within
you heals your sick self.
Therefore trust the physician, and drink his remedy
in silence and tranquillity:
For his hand, though heavy and hard, is guided by
the tender hand of the Unseen,
And the cup he brings, though it burn your lips, has
been fashioned of the clay which the Potter has
moistened with His own sacred tears.
This is the most complex metaphor in the poem. The Potter, who made the cup in which the physician keeps his potion, is a  personification  of “the Unseen” in the last line. Note that  the word “His” is capitalized , a convention from religious texts that strongly suggests identification with the Abrahamic God or a similar figure. 
 
 If the potion is pain and the physician is your soul (your instinct), what is the cup? Most likely this is an example of  metonymy , using one word to refer to something else, and “cup” is synonymous with “potion,” i.e. the pain he brings. The pain (the cup) was fashioned by God Himself. 
 When you use a potter’s wheel, you need to moisten the hard clay in order to make it malleable. The  sacredness  of the tears seems like a given, but why has God/the Potter used His own tears in the first place?  What is being implied here about the nature of God’s crying? 
 I think it likely that Gibran is saying  God cries for you , too, that He is concerned for you, that he has shed tears over the necessity of your suffering. But it is just as likely that  Gibran’s Unseen suffers as much as you do , for His suffering is the sum total of all of the suffering in the known universe: His tears mirror yours. A sense of camaraderie, then, is perhaps present. 
 
 I really don’t want to overanalyze here, as it might just do the nebulous quality of the lines a disservice; rather, you might heed St. Augustine’s admonishment in  On The Gospel of John : 
 Weigh the words, and get a knowledge of the mystery.
On Pain