Prison my heart in thy steel bosom's ward, (9-12)
Our speaker spends the entire third quatrain suggesting that he feels like he's been imprisoned in his mistress' heart (a.k.a. her cold "steel bosom"). That's pretty hardcore. It's also a pretty effective way for him to show just how powerless and helpless he feels in a relationship that he can't seem to get himself out of.
And yet thou wilt, for I, being pent in theePerforce am thine, and all that is in me. (13-14)
Here, our speaker just sort of gives up and comes to accept the fact that his mistress is never going to stop torturing him or his friend. That's because the speaker feels like he's "pent" (imprisoned) in her. Dang.