He doesn't want it to end either. Keltham can see it, or thinks he can, that Carissa is just existing now, with some of the noise in her head quieted, and he wants to protect her from all her thoughts, and extend the protective time she has where she doesn't need to feel them. It feels right, what they're doing now, the way he's possessing her and the way she's being possessed.
Keltham has a Lesser Restoration or two queued if at any point he starts to feel fatigued, or she does (as shown by her reactions starting to diminish). He decided before this started that he'd play with Carissa however he felt like it, until he stopped feeling like it. He's unlikely to stop feeling it, so long as Carissa looks - safe, and protected, and happy, like this. That's the way a possession of Keltham's should look.