I think there must have been a period of time between when Solas decided to tell Lavellan who he was and the Crestwood break-up scene.
I think there must have been a couple of days or maybe even a couple of weeks where all Solas felt was this sense of relief. He is going to tell her. Everything. He is going to rid himself of this lie that exists between them, that keeps them apart, and just be completely honest with her.
Maybe, during that period, he was more free with his affections than he had ever been before. Because he has decided to let go of this thing that was keeping them apart. Maybe he does not even care what she'll do with the truth after she has it, as long as she finally has it.
He thinks, I will tell her the truth and she will save me. Even if she stops me, she will save me. And he's so happy in that period just before Crestwood, and he kisses her freely and holds her and he's just more expressive with his feelings than he ever has been, because this weight has fallen away. He's going to tell her, and whatever happens, happens.
Then at Crestwood, he's holding her hand, walking with her like they've done that a thousand times. They're in an established relationship at that point. Game mechanically, the romance lock signifies the point of no return and, when Solas breaks up with you, that it is the point where there is no backtracking out of this relationship anymore. You can't untangle your hearts after that. Its not a commitment, its the opposite in fact, but I don't think Solas ever loved Lavellan more than he did at the moment when he realised that, because he can't bring himself to tell her the truth, he has to try and let her go.
And I think that the period of blissful hope just before Crestwood makes the despair they feel afterwards all the more profound. It all comes crashing down, so fast.
I distracted you from my duty, Solas says, knowing full well that she has distracted him from his.