There's a saying Katherine heard a lot growing up. She doesn't remember the exact wording, but it went along the lines of how telling it is about how someone deals with loss. Losing with dignity...never mind, it was a sports thing. But part of that had to be true, right? How does she react to losing it all?
For a while, there was a painful, gaping void in her chest. She didn't know how to function without Dean. She knew she did it just fine before him, but after Dean? She couldn't even conceptualize it. But she learned, eventually, that it wouldn't go away. It just went numb, masked beneath sleep, back-to-back hunts, and bar hopping.
Even after his return, Katherine couldn't quite find the tools to fill that void. She knew how to cover it up, though. Plaster a smile on her face, try a few jokes. She tried to get back to the girl everyone seemed to love so much, but she hated that girl--because no matter how hard she tried, she couldn't find her.