Lord, who createdst man in wealth and store, Though foolishly he lost the same (1-2)
Although Herbert doesn't go out and shout SIN NO MORE to Adam, he makes it clear in stanza 2 that Adam's mistakes are more than just foolishness: they're genuine sins. Why else would every subsequent human be born into "sorrow" (11)? Because Adam committed the first sin against God, everyone else inherits it and must figure out some way to chuck it. For Herbert and other good Christians, that means using Christ's resurrection to repent.
Decaying more and more, Till he became Most poor (3-5)
Although "decay" usually refers to bacterial decomposition of organic substances (see: bananas), in the context of Adam's slip-up, it also means moral decay. That forbidden fruit might have tasted good and all, but now Adam's soul is rotting, so thanks for nothing.
My tender age in sorrow did begin (11)
There's a lot of blame in this one line and it's directed towards Adam. If he hadn't sinned, maybe the speaker could have been born in joy or glee or even just moderate okayness. Instead, every human comes into the world with Adam's baggage, or what Christians call original sin. Not only does that mean they have something to repent right off the bat; it also signals that they'll confront a life of hard work and pain.
Thou didst so punish sin, That I became Most 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.