Thou didst so punish sin,That I becameMost thin (13-15)
According to the poem, God's job is to punish sin, whether it's long-gone Adam's or the just-born speaker's. His thinness here is metaphysical as well as physical, a thinness of the soul. Where is God's love when there's sin around? Nowhere, that's where.
Affliction shall advance the flight in me (20)
Adam's sin means that everyone after him starts out with a stacked deck. The speaker's additional sins have only increased the odds, but since he's willing to repent, the speaker actually sees this as a good thing. It's all about the mindset, folks. Remember when your teacher used to correct you for saying "have to": "No, you get to complete this math assignment"? Herbert does something similar here. The speaker's sin means that he gets to use Christ's resurrection for repentance, not that he has to.