Gotta add my view to the "What is Solas' true plan and motivation and regret?" theory.
Solas hints many times that Ancient Arlathan society, nobility and/or "gods" were little better than Orlais or Tevinter; that there was corruption, tyranny, slavery, politicking and court intrigue just like now. Bring him to the Winter Palace and he mentions afterwards that he forgot how much he missed the dangers of court intrigue. Since we know who he is, he probably experienced it in Ancient Arlathan. When he reveals the truth of the vallaslin, that they were slave markings nobles carved into their slaves to honor their gods, Lavellan can lament that the Dalish try so hard to remember what once was, but they wind up holding onto relics of a time "no better than Tevinter."
The game also hints many times that Solas deeply regrets something he did in the past (which Cole touches on). That he's experienced war and intrigue and considers himself something of a veteran like Blackwall, who asks if he was involved with "some elven skirmish," which Solas cagily says he would not have heard of. Since the Dalish have pinned him as the one that trapped the Elven Gods (which Cole touches on when he mentions "they sleep, hurting"), and he strongly implies that he's done something to make this future, maybe...
If you drink from the Well of Sorrows and Solas asks you what you intend to do with it, if you say you want to make the world a better place, he rather heart-wrenchingly asks you "what if you wake up in the future and the world is worse than the one you sought to change?" If you say you want to rely on the advice of others, he says that while individuals help people, "groups never do." He feels that groups and organizations inevitably become corrupt and self-serving, and mentions that he knows firsthand how frustrating to see fools undermine all the good you sacrificed to obtain.
However, if you say you want to undo the damage Corypheus has caused and set the world straight again, he approves. EDIT: He seems the most cheered, and even smiles, "Perhaps our past can be restored."
It seems that Solas no longer believes in lofty ideas like "change the world for the better" or "rely on groups / organizations to help people," but he does believe in fixing a specific problem that had created a specific outcome. Which makes sense, when one keeps in mind the whole reason he's involved in the game is to try to undo his specific mistake of giving Corypheus the Orb. And before that, it's implied that the reason he gave Corypheus the Orb was to use it to fix a specific mistake he made in his past to create the current present. (Which might have something to do with the absence of the Elven Gods, which the Dalish pinpoint Fen'Harel as the culprit, which might have something to do with his many hints that they were like Orlais or Tevinter, which may or may not tie into Mythal's betrayal and murder.)
Add all this to the Stinger, where he tells Mythal that he knows he should pay the price for giving Corypheus the Orb but the People still need him, and it seems that he's determined to fix whatever mistake in the past he attributes to the world's current problems and believes he must do it alone; not rely on the Inquisitor, Inquisition, or anyone else.
Okay, well, maybe that was a little obvious. I just suspect that Ancient Arlathan's corruption as well as it's possible destructive war with the Forgotten Ones and/or it's possible connection to Mythal's betrayal and murder, might have motivated his past mistake. It's possible that in the past he might have thought the common elves of ancient times would be better off without the elven gods' politicking and power-play holding them down (slave-markings and whatnot) and did what he did because he wanted to help the People, but somehow slept through what happened next and "woke up" to find the world was worse after what he did than before, and wanted to use the Orb to fix it. Now that the Orb is broken, on to Plan B.
*exhales deeply*