My speculation is that Solas/Fen'Heral sealed away the other elven gods, perhaps with the intention of protecting mortals from the coming clash between the two groups of gods. We know from the legends that the two groups were nearing war with one another, which is when Fen'Heral tricked both sides into remaining on their 'side' of the world. It is also quite likely that the elven gods might have fostered some of the corruption in the old elven rule, such as the rigid caste system and slavery. If Fen'Heral is the god of rebellion, it would make sense that he eventually wanted to break the system.
But by trapping the gods on one side and the Forgotten Ones on the other, Fen'Heral has been in the middle. Over time, perhaps he's grown to regret the decision. His fear says that he is afraid of dying alone. His only peers would be the very people that he sealed away. Elven society also collapsed in upon itself without the 'protection' of the other elven gods.
To Solas, it might seem very much like he made a mistake. He betrayed 'his people', and he's possibly forever alone. And what he did didn't even help the elves, at least from his perspective. They are now mortal, and their society is effectively in ruins.
Cole: They are not gone so long as you remember them.
Solas: I know.
Cole: But you could let them go.
Solas: I know that as well.
Cole: You didn't do it to be right. You did it to save them.
Inquisitor: Solas, what is Cole talking about?
Solas: A mistake. One of many made by a much younger elf who was certain he knew everything.
Cole: You weren't wrong, though.
Solas: Thank you, Cole.
Then there is also this conversation after the break up:
Cole: 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... (gasp) Where did it go?
This makes me think that the elven gods are the ones sleeping in the mirror. But to wake them would be a bad thing, and its very possible mortals would be the ones who would most suffer if all of a sudden the elven gods [and forgotten ones] were unleashed. If he has a connection and sympathy to a mortal, then his decision to unleash them becomes extremely complicated.
'It changes everything but it can't', to me, seems to mean that although Solas is guilty about his plans, he still plans to go through with them. Which will likely play into Sandal's prophecy of all magic returning. And everything else that has been sealed away.
This is only my random speculation though, not anything I would say 'this is what is happening fullstop'. Something to muse over, though!