The real answer, of course, is that he can't, not really. She can choose to believe him or not, and isn't that the founding choice in every relationship?

Maybe if she were a better, less selfish person, she would set him free to see if there's anything left of those old feelings left to spark. But she's not better. She's selfish. She wants him for herself.

And she thinks if she tried, it would only hurt him terribly anyway. "I believe you," she whispers, and brings his hand to her chest, over her heart. "It's okay, Bucky. I believe you. Just be mine, the way I'm all yours. That's all I want."