“only you”

I think that Heisenberg would feel guilty for a long time that he‘s taking care of Rose when he literally planned on using her as a weapon, and he knows that even though Ethan is dead, the man would NOT approve. So he would always refer to himself as "Karl" to Rose, and discourage her from calling him any variation of "dad". He would actively think that he's doing good: Miranda forced him to call her "Mother" when he didn't want to, so he's sparing Rose from what he went through.

Until this moment.

He'd finally realize that he's been simplifying it too much, and Rose's situation is fundamentally different than his: he didn't trust Miranda, but Rose trusts him, and feels rejected when he scoffs at the idea of them being a "family". And if anything, him dismissing her true needs in favour of what he thinks she needs, is something Miranda did constantly to him.

Idk how much of this comes across in the comic itself, hopefully enough!