Wilt thou forgive that sin which I have won Others to sin, and made my sin their door?
Yikes, now he's making other people sin. What's with this guy? And how's he even doing that?
Well, by sinning himself, he's making a kind of "door" for others to walk through and sin in the same manner. Once a door is open, it's a lot easier to walk through. (Oh, and he's using a metaphor here; he doesn't mean an actual sin door. That'd be just… weird.)
This sounds kind of like peer pressure, right? Or, maybe other people just thought the speaker's sin looked really fun and decided to copy him.
Whatever the case, the speaker still wants forgiveness for encouraging these sins, even if he didn't mean to.
Wilt thou forgive that sin which I did shun A year or two, but wallow'd in, a score?
He also wants forgiveness for the sins that he resisted for a few years. Man, this guy is thorough.
Ultimately, he indulged in these sins, "wallowing" in them for much longer than he resisted them.
A "score" means twenty years, though the speaker probably isn't being literal about the amount of time he spent sinning. He's contrasting the time he spent resisting the sin to the time he spent enjoying it.
As for the form so far, we're back to the same rhythm and rhyme we saw in the first stanza. Donne isn't changing it up much.
When thou hast done, thou hast not done, For I have more.
Welp, here we go again. Just when God thinks He is done forgiving, the speaker has more sins planned out. He's like the Energizer Bunny of sinning.
Notice anything familiar? You should. These lines are the exact same as the two lines that end the first stanza. That's what we call in the poetry biz a refrain.
This repetition has a particular effect: it reminds us that, no matter the amount of forgiving God can do, there's always going to be more sin. It all goes back to human nature, which Donne emphasized in the first stanza.
Notice that the word "done" is similar to Donne. Coincidence? We aren't sure, but Donne was a meticulous poet, so we are betting not.
In fact, Donne is considered one of the Metaphysical Poets; they were famous for using puns and wordplay to make their poems intricate (and often a little mysterious).
Perhaps, then, in naming himself in the poem, he is equating himself with the speaker. While it's always iffy to confuse the speaker of a poem with the poet his- or herself, if you try replacing every "done" with "Donne" in the poem, it quickly becomes a poem about Donne's mortality.