I don't believe he set out with the intent to break up with her. He took her someplace private and special to tell her how special she was to him, and he might have planned to tell her something more than just about the meaning of vallaslin. But, he obviously felt from the beginning that romance was a bad idea (forever reluctant). I wonder if he broke it off because he happened to realize that if he kept going, he was going to have to explain things to her. If she took it well, that wouldn't have been a bad thing, but then it also might endanger her. But the problem is, once he explained, "My heart, I am actually Fen'Harel," how well do you think that's going to go over with a Dalish elf? It doesn't matter what lore she'd uncovered, her gut reaction would probably be unpleasant to say the least--assuming she managed to believe him at all. And, others are right--at that point it may have been a matter of continuing the relationship or moving forward with his plans, either due to danger to Lavellan or simply because he didn't feel she'd be able to help. And then there's the whole "he's immortal, and Lavellan isn't" deal. I do wonder what would have happened if he'd gotten his orb back, but considering it appeared to be corrupted by Corypheus at that point, it might be for the best that he didn't. And yet, perhaps, if he had, it would not have been the end of their relationship either.
Here's another thought for you, though... Solas is Fen'Harel, an ancient elven god. Even assuming Lavellan can handle his being Fen'Harel, how is she going to take his being a god? If she's faithful to her people's religion, then that puts him into a position unimaginably higher than hers. What right does she have to be with a god? Solas might have had to consider that, too. It would have complicated things, and, given the Inquisition's Chantry ties (like it or not), might have automatically put him at odds with the Inquisition and definitely with the Chantry, regardless of Lavellan's personal wishes.