Lying by omission is still lying in my book.
It's not lying in my book. "Lying by omission" is such a misnomer it angers me. There are plenty of times when not telling the whole truth is completely justified and also the best course of action... which honestly I think is Solas's case, as many have pointed out. If he just came up and told the whole truth, there's no way that would have gone well. People would be too focused on dealing with Solas/Fen'harel instead of dealing with the breach.
I guess it all depends if you consider lying by omission a lie ? You dont need to lie to decieve.
He does put everyone's life at risk, and specially Quizzy, by not telling the truth about things like Cory, the Well, etc.
How did he put anyone's life at risk any more than it already was? The breach would still be there, whether he told the truth or not. They would still have to stop Cory from getting the Well, whether he told the truth about what it was or not. Nothing would change other than maybe your choice to drink/not drink, but he still tries his best to dissuade you.
He could have told the truth in a different way, instead of keeping it all to himself. [snip]
He did tell the truth in a different way. He told the truth about everything other than how he came by the knowledge.
To be fair, that's different. Lying about Santa and lying (by omission) about your very identity to someone you love aren't really the same.
And in that comparison, it's as though Solas views Lavellan as a child who can't handle the truth/needs to believe a lie in order to be happy.
As far as Solas viewing Lavellan as a child: I'm not sure I agree. I think that is a part of it, but more from his fear of how you might react. Even so though, he constantly says how you surpass his expectations, how you surprise him. I don't think he would believe you incapable of understanding. I think it has more to do with the fact that he has unfinished business. At the point of the breakup, you are just about to face Cory. Assuming you live, Solas is thinking he might have a chance at getting his orb back, and so has to go do whatever it is he had to do. Part of me thinks he was planning to sacrifice himself in some way, due to the way he talks to Flemeth at the end "I should be the one to pay the price". So in this case, he breaks up with you because he wanted to spare you the (possibly worse) heart ache of having him die on you. Either he was going to die, or have to go back to sleep again, either way, kind of terrible for you.
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.
[snip]
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 
I don't think he only feels bad if romanced. In one of my pt I did not romance him, but still had max approval, and I could tell at the end he still felt terrible. Mostly because all of his scenes are nearly identical minus the smooches. He even says at the end "No matter what happens, I want you to know I respect you". Definitely some huge guilt there that he had to play everyone.
Re: vallaslin: The way I play my Lavellan, it is in no way a symbol of her identity. You can play it that way, sure, then don't have him remove it. But I see it as a relic of something that they don't understand, and also after seeing the way god-worship can go so wrong, probably doesn't really want anything more to do with it. I guess my point is: if you think your vallaslin is your identity and Solas is wrong to remove it, there's an option for you to say no. I don't understand getting miffed about him removing it and leaving, like "he tricked me!" when he still told you the truth and it was your decision to let him remove it.