"You’re real, and it means everyone could be real."
Imagine you’re Solas for a moment, really think about how Solas must feel carrying the weight that he does. It’s an ancient pain, this burden. Once upon a time he was a hot-blooded young elf, who thought he had everything in hand, who thought he had all the answers. He did something huge and world-changing, not to be right, but to “save them”. Once upon a time, Solas stood tall and thought he Knew.
Now he’s woken up, and the severity of his error must have hit him so unbearably hard. The sick tendrils of his mistake permeate every facet of Thedas that he turns his gaze to, both in the waking world and in the reflections of the Fade. Now he stands alone in the ashes of what his pride has wrought.
I don’t see it as being too far-fetched that he ends up with coping mechanisms. He’s really quite reserved and distant - distant to the other companions (see how he does not join in the game of Wicked Grace), distant to the modern elves, distant even to a romanced Lavellan. He spends a lot of time wrapped up in the Fade and focused on the past. Part of how he deals with the sheer enormity of what he’s done, I sometimes feel, is distancing himself from the Now. This applies mentally. I think it’s easier for him to cope with what’s happened if he views it disjointed, as if from a distance, like he’s depersonalized - like it’s not real.
The world Solas has awoken to is literally a waking nightmare for him. It isn’t real, it can’t be. These elves of Now are not his people. They aren’t real, none of this is. He holds everyone at a distance and puts up barriers and looks elsewhere.
He has to right his wrong, and the elves of now may suffer for it. It’s easier to deal with the guilt of that thought if they don’t seem fully real to him.
But Lavellan blazes so brightly that she gets through some of his barriers, shatters the frosted glass, enough to at least make him see her without a foggy haze hanging between them. And if she’s real, it means everyone else could be real.
And it changes everything, but it can’t.