And yet thou wilt, for I, being pent in thee Perforce am thine, and all that is in me.
Here, our Speaker comes to the sudden realization that, no matter what he does or says, his mistress is never going to stop torturing him and his friend.
This sudden shift, by the way, is called a "turn" or a "volta," which is just the moment in the poem where the theme or the tone changes in a sudden and surprising way. Go to "Form and Meter" for more on this.
Up to this point, our speaker has spent the entire sonnet bawling out his cruel mistress and trying to come up with ways to get her to stop hurting him and his friend.
The big tip-off that our speaker is now switching gears is the phrase "and yet."
This is where it suddenly dawns on him that his mistress isn't ever going to change her evil ways and he just sort of throws in the towel.
Because he feels imprisoned ("pent") inside of her heart, and he belongs to her, "perforce."
"Perforce" means "of necessity," so he's saying she totally owns him and that's just the way it is. Did you notice how the term "perforce" has the word "force" in it? It reinforces the idea that the speaker is completely powerless and that his mistress has got the upper hand.
And it gets worse.
The speaker says that, since his mistress also owns everything "in" him, that means she owns his friend, too. (Remember how the speaker said he was going to protect his friend inside of his heart? The idea is that that the friend is near and dear to the speaker's heart.)
Not quite. There's also another sex joke here.
When the speaker says he's "pent" inside his mistress, he's also making a reference to what it feels like to be physically inside of her.
Okay. So, why's our speaker comparing his mistress's vagina to a jail now? (It's bad enough her heart has to be a "prison," right?)
Basically, he's saying that he is totally powerless (not just emotionally, but physically, too) when he's with her, especially as he climaxes or, gives up "all that is in" him.