This was one of those situations, where Bucky could recognize that they were walking in on the middle of the start of an emotional breakdown. Natasha clearly had been going over her thoughts over and over, probably having multiple thoughts at once all crashing into each other and causing a major emotional overload. He could relate, in a big way, having gone through plenty of those himself in the last handful of years ever since regaining his identity and breaking free from HYDRA. It wasn't the same, but it was similar. Enough that he knew that the best thing they could do right now was just let her talk. Let her get out everything she'd been harboring for so long, carried silently on her shoulders, avoided like the plague as long as she could.
He knelt down and kept his gaze focused on her. "There's no winning here, Natalia," he said. "You just need to feel what you're feeling and know that we're here." He'd listened to every word she said and he didn't try to reason with her, didn't try to rationalize or brush any of it under the rug. These were her feelings and they were valid no matter what. He did think it was a good thing she was starting to unleash at least a little bit, even though he didn't want such pain for her.
"You say what you need to, you feel what you need to. It's not going to drive us away." He reached up with his other hand and sifted through the ends of her hair.