Of course she knew. She had made that particular bit of information clear to him in ways great and small in his time knowing her. The nightly nuisance that become his partner, his friend, and ultimately the girl he cared for most in all the world. These were things he knew, and had known for some time now, but now that his father was gone he felt even more drawn to her. Almost as if she was all he had left, even though that was not... quite the case, given his close association with the rest of the Bat family and Bruce in particular.
He drew back, and realized that for once he was the one without the mask while she still had her own. In a way, maybe that would make this easier. Not having to see her.
"There's more Steph, and this isn't easy for me to say, especially to you, but I can't do this anymore." He plans, to be sure, but he did not know how to explain them to her, other then to make it known what he was about to say. "I can't be Robin."
There was more to it than just the death of his father too, but he did not care to explain any of that now either.
"...And I'm planning on leaving Gotham. At least for a while."