The speaker curses his mistress (a.k.a. girlfriend) for wounding his heart and the heart of his BFF. While he's at it, he manages to sneak in a dirty joke about how she's been sleeping with them both. (Love triangle? Check.) He says it's bad enough that she gets off on torturing him, but now she's gone out and turned his "sweet'st friend" into her pathetic love "slave," too. Dang, girl.
He tells her that he's not even his own person anymore because it's like she totally owns him. What's worse, she's got his buddy even more whipped. This makes our speaker feel like he's been abandoned by three people: 1) his friend, 2) himself, and 3) his mistress. He says this is the WORST. TORMENT. EVER. (OMG! Did his mistress stop seeing him? Has his friend stopped talking to him? Or, does he just feel emotionally abandoned? We're not sure.)
Next, he imagines that his mistress' heart is a cold jail cell that's holding his heart prisoner. (Translation: dude feels like a prisoner of love.) Then he imagines that his heart is also a little jail that's keeping his friend's heart locked up safe inside. (Yeah. That's a little weird and confusing but, then again, so are love triangles.) He tells his mistress to go ahead and do her worst to him, because he just wants to protect his BFF from all the mean and sadistic stuff she does.
But then our speaker suddenly realizes that his mistress is just going to keep on torturing him and his friend and there's absolutely nothing he can do about it. Bad times.