He did know the other ones, at least he was pretty confident that he did. That is to say, he knew about what she'd been through all the way up until they were separated. And he'd pieced together some of what she'd gone through afterward. Bucky trusted that if there were other things that she'd tell him, just as she'd talked to him about all of this. Bucky loved her as she was. He didn't expect her to be better or anything like that. For god's sake, he was usually some kind of disaster and Clint had his share of issues. None of them were perfect, but they all knew how to balance each other's imperfections and help each other. Even if they didn't know what to do or say, it somehow managed to work itself out to being the right thing.
Being in this place, having developed the relationships he had and knowing what was to come should he ever end up back where he came from, Bucky knew all too well what he stood to lose. Nothing felt worse than the idea of going back to a place where for one thing, Clint wasn't there, and for another he was destined to be reduced to dust only to come back and find out that Natasha had sacrificed herself. Nothing she could say, nothing Clint could say, and nothing either of them could do would be worse than that reality. Keeping that perspective, Natasha could've said pretty much anything.
"A whole few minutes? We need to work on your stamina, lisichka." He smirked a little. That super soldier serum kind of came in handy in that regard for his anatomy. "I've been trying," he said, more sincerely. "Trying to live solely in the moment because I know that's all we have. But sometimes, I don't know. I'm afraid everything is too good to be true. I'm happy, being with you and Clint. It's so unconventional but it works, doesn't it? This all works. And nothing has worked for me, ever. Not relationship wise. So I just keep feeling like I'm going to mess it up and I keep trying not to go there, not to get into my head."