«That thou hast her, it is not all my grief,
And yet it may be said I loved her dearly».
Only in this last sonnet concerning the youth and the poet’s mistress does the poet make fully apparent the main reason for his being so upset: “That she hath thee is of my wailing chief, / A loss in love that touches me more nearly.”
That thou hast her, it is not all my grief,
The poet is grieved by his mistress’ infidelity, but he laments even more the fact that she has what he so ravenously craves: the physical and emotional attentions of the young man.
Reconciling himself to his mistress’ behavior requires all the poet’s powers of expression and self-deception. He makes the torturous argument that since he and the youth share personalities, they must share the same woman: “But here’s the joy: my friend and I are one; / Sweet flattery! then she loves but me alone.” Likewise, because the poet loves the woman, and because the woman is having an affair with the young man, then the rational conclusion — according to the poet — is that the poet and the youth are that much closer in their relationship.
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com