My interpretation of the balcony scene is that Solas by this point has fallen completely and hopelessly for Lavellan, but up until the moment he asks to speak with her privately has been trying very, very hard to keep it under wraps. He knows it's a bad idea because of
~reasons~, but he can't help but finally approach her and put the ball back in her court. I imagine he wouldn't bother with the last ditch attempt of staving off the relationship by putting the ball in her court at all if he weren't conflicted (which stems from the general nature of
~things~), but Solas' life is clearly complicated at this time, which affects the way he behaves towards her, even though he wants her badly.
When she moves to him after he says he hasn't forgotten the kiss, the way he moves--kind of padding, shifting his weight, pulling away just a bit--is to me demonstrating that he's still trying in some small way to resist, as stupidly half-hearted as it is, and is most certainly conflicted, and perhaps also just plain old nervous or uncomfortable. He did say things have always been easier for him in the fade, after all. And that it had been a long time.
At that point he uses all of his remaining self control in one final bid to leave, stopping anything from happening, which he knows will push him past his tipping point, and when Lavellan stops him, tells him not to go, he is catapulted past that point of no return and gives in entirely, losing himself despite everything in his brain yelling that it's a Bad Idea. He kind of lays himself bare before her, the way he absolutely throws himself at her, enveloping her in one hell of a display of passion, and in the moment is so overcome by finally being able to express himself that he reverts to his first language and spills the big can o' beans that he loves her. Love beans.
At this point, I think he internally panics a little bit, or perhaps is just too overcome and is like, holy ****, I cannot actually deal with my emotions or this situation or this amazing woman right now, and leaves before she even has a chance to catch her breath. We know he chickens out with telling her the actual truth during their final scene, and I feel like he's chickening out here a little bit as well. She'll react in some way, and he's likely scared of that reaction, for many reasons. I imagine he's kinda like, **** **** ****, what have I done, have to leave, have to leave noowwwwww. *wipes sweaty hands on thighs repeatedly while pacing in the rotunda*