This is an interesting sonnet, especially from Shakespeare who seems to have had an aversion to oaths throughout much of his works, both plays and poetry.
In the first four lines, the poet begins by acknowledging that he has broken an oath ("I am forsworn"), which is important because oaths were usually taken very seriously and often carried a religious connotation--as in, "I swear before God." In this case, the poet, by loving the person in this sonnet, has forsworn himself. But he immediately accuses his lady (assuming it is a lady) of breaking two oaths: the first is her marriage vows, and the second is her vow to hate her new lover after vowing to love him.
The poet then says, in lines five through eight, that blaming her for breaking two vows seems ridiculous when he has broken twenty vows in order to abuse her: he has sworn an oath that attests to "thy deep kindness," which we are meant to understand violates the truth. Further, additional oaths relating to her constancy in love, her truthfulness, are also false.
The last four lines carry the effect of forsworn oaths to their logical conclusion. When the poet says that "to enlighten thee, gave eyes to blindness," it's quite likely that he purposely closed his eyes to her negative traits in order to make her look better or, even worse, made his own eyes deny the truth about her that they clearly see.
The couplet contains the word "perjured," which adds a legal dimension to the forswearing of oaths. The poet affirms that when he swore "thee fair," he not only swore "against the truth," but also committed perjury. Oath swearing is a moral construct whereas perjury is a legal construct and opens a whole new group of problems for the poet.