Castiel knows the difference between what is real and what is Lucifer’s doing.
What’s real is that he is back to being alone, contemplating his mistakes and poorly-spent long life. What’s real is that he is in this situation because of himself, not because of Sam Winchester. What’s real is that Dean would have stopped him if he knew beforehand what was going to happen, even if the righteous man would have regretted every second of it if Sam died consequently.
What isn’t real are the flames melting his vessel’s skin, climbing the walls of the overly clean room. What isn’t real is the way Lucifer winks and touches Castiel late at night when the orderlies have all gone away. What isn’t real is him secretly craving that touch—any touch for that matter—after being stuck in the institution for weeks. What isn’t real is the anger he holds when he thinks of Dean leaving him here, alone and suffering, with no one he can really speak to.
Or maybe all of it is real.
It makes no difference to Castiel though because Sam is safe, Dean is relieved, and he is paying for what he’s caused. This is the hand he’s been dealt, and nothing in the world could change that.