Okay, so the story goes like this:
Kagome gets kidnapped and is turned into a demon by this guy who wants revenge against sesshomaru. So he uses a piece of sesshoumaru, strands of his hair, and turns her into a silver dog demon. Sesshomaru finds her in a cage, and is automatically smitten with her. Once rescued, kagome tells sesshomaru that he's being tricked and that she's been turned by a demon, who swears he can turn her back, in a plot against him, but that she had no choice in the matter. Sesshoumarus beast is constantly telling him that she's his mate because the piece of him they used has turned her into his ultimate dream girl, or something like that, but his more logical side is fighting against instincts. Eventually, they both agree to continue on the plot of sesshomaru not knowing about the plot against him in order to catch the guy. One of the ways to do this is to initiate a courting ritual, one of several. The one sesshomaru chooses is the training of the mate.

I can't remember much else. Please help. I'm going insane trying to find this fanfic.

Sounds like Exacting Change by AccidentalGoddess on Single Spark (R there, M here0)
www.dokuga.com/spark/story/2861/1

They say the sins of the father are visited on the sons...but why in the world did they have to drag her into it? Someone is after revenge and SOMEHOW Kagome gets dragged right smack into the middle of it. Now they're stuck in a game no one knows the rules to with a plot devised by a madman. And somehow they have to bring it all to an end before Sesshomaru goes mad!

EDIT BY WICCAN: Actually, due to language, this story would be rated MA here on Dokuga, NO YIMs!!

Thank you! That was the fix I was looking for. It's sad that the author hasn't finished or updated it in a while. It's such a lovely story, and I'm so eager to see where it goes should they update it.