could it be this one from 2013, Repent of Your Sins, http://spnkink-meme.livejournal.com/76256.html?thread=27488480&#t27488480

When Cas returns to Dean and Sam, human, confused, disoriented and scared, they take him in. What else can they do?

But Dean is still harboring some resentments. Sam is still sick/damaged from the trials, and Dean feels that Cas is to blame for much of what's happened to them in the past couple of years - things he's never really made amends for.

Dean decides to give him the chance to "make amends" - only Dean's idea of this involves taking out his frustrations on Cas, slapping him around (taking advantage of the fact that he physically can now that he's stronger than Cas), emotionally berating him and making him feel like crap (constantly reminding him that it's his fault this is happening to him) and possibly even using/abusing him sexually.

Cas feels so guilty about everything, and is also so scared and incapable of taking care of himself as a human, he feels he has nowhere else to go. Some Stockholm syndrome type themes would be welcome if you want to go there, with Cas feeling he both owes Dean and needs Dean and being too scared to leave him.

Thank you so much, that is exactly it! :)

Hey, got you the AO3 link, so it's easier to read. :)

http://archiveofourown.org/works/1178489/chapters/2402704

Thank you so much, that is SO much better! :) <3

:)