By the time you get to the Temple of Mythal, Solas got caught in his own game. He knew what the damn Well was about and said nothing.
Solas had enough time during the course of the game to explain he is 'an ancient elf' or 'something something spirit' to the Inquisitor so he could use his knowledge to help out without having to go 'hey, im a god and thats my orb'.
Maybe if Inquisitor is on good terms with him, he wanted to keep the experiment going, if on bad terms then he is afraid for his own safety, and if romanced then he didnt want to mess things up (until he has to).
But still, he got caught up in it and fell short.
And the only way you get the notion he feels bad about the way things have been going is if romanced.
I dont think he breaks up because he wants to protect Lavellan from following on his big mission (usually LIs that have that in mind just leave without breaking up, with a letter saying 'Dont follow me' to make sure you'll follow).
I think he breaks up because its hurting him to keep lying (by omission) to Lavellan, even if the romance its not hurting her (because Lavellan keeps wanting more until the breakup). He could have kept playing the part to make Quizzy happy and pretend 'big mission, have to go' at the end.
He had her in his hands, to a point she would let him take the Vallaslin away (symbol of her identity).But he actually made a conscious choice to stop fooling Lavellan, and lose his precious 'sidebenefit', because it was wrong. So maybe he is capable of changing his ways ?
Is it great for our Lavellan? Not really. But it is a damn good story 