None of this will make sense unless I start by saying that I think ancient elves could be spirits of some kind, either body-riding or manifesting temporarily in the physical world and retreating to the Fade in uthenera when they could no longer sustain themselves. The spirits/First Children mimic life so I think after the creation of the Veil they entered the physical world as god-like beings/a sort of nobility, and possibly those the Dalish consider gods and the forgotten ones are ancient elf nobles/spirits who fought over something. That something might've been an attempt to actually cross into the physical world and become real, which could've somehow began the Blight. The Chant says the darkspawn find old gods and corrupt them, but maybe in crossing into the physical world the ancient elf spirits became corrupted (just as a spirit of Wisdom can be corrupted into pride).
Mythal comes to Flemeth as a wisp, and think about things Solas says. He constantly refers to the Quiz' spirit, all his friends are spirits, he talks about slipping across the Veil with a thought, he remembers a time when elves could change the world with a simple gesture (sounds like the "formless, ever-changing" world of spirits from the Chant of Light), and then Cole has some mysterious things to say about someone who might be Solas which make him sound quite spirit-y:
"Old pain, shadows forgotten from dreams too real. This side is slow and heavy, but here is what can change." and "Wisdom knows enduring is pain. He hurts for her, another of many he couldn't save. He carries necessary deaths."
This brings me to Fen'harel/Solas' original purpose. The attempt to become real could've threatened the ancient elf commoners (the whole world?), which is why Fen'harel sealed the 'nobles' away somehow (killed in uthenera possibly). He thought he was creating a better future where the elves could thrive without the nobility, but when he awoke he discovered the future he created was worse than what existed before, that's why he is initially so upset with the Inquisitor after Mythal's temple when she says she wants to make things better. In a banter with Sera he pushes her to replace the nobility with some new power structure but she says if you lop off the top something just as bad will take its place, this is when he finally agrees with her, maybe because he took out the slave-owning ancient elf nobles and they were replaced by Tevinter slave masters? However, when he returned to the world he sought out the elves (in interactive banter at the beginning of the game he says he tried to teach the Dalish but they attacked him), so he could've thought that bringing back the ancient elf spirits was better than losing elven culture altogether.
So that said,
"He hurts, an old pain from before when everything sang the same. You're real, and it means everyone could be real. It changes everything, but it can't. They sleep, masked in a mirror, hiding, hurting, and to wake them..."
Before this part Cole refers to the Inquisitor as 'her' and not 'you.' If you encourage Cole to be real Solas says he never thought he'd witness that and he dodges a question from Cole about having seen it in the past. I think after being disappointed by the modern elves and the world he helped create Solas decides to bring back the old ways by tearing down the Veil (he thinks that'd be a marvelous world), but finding out that Cole, a spirit, could become real and truly live and experience all that life has to offer throws a wrench in his plans. (falling for the Quiz and learning that modern elves may be redeemable also does, but the process of spirits becoming real is a more pressing issue) "...everyone could be real." If the spirits he loves could achieve true life then he'd have to question everything he'd been striving for ("it changes everything, but it can't.") Also the "sang the same" is terribly important but I could write forever on that subject >.< And yes, Solas wanted to seal the Breach and strengthen the veil but I think that's because Corypheus' methods were going to destroy the entire world instead of integrating the two, and that'd be awful for everyone.
Now I'm not totally sure what Solas' goal is at the end and I'm tired of typing so I'll stop there 