It wasn't her fault.
Ghosts controlled their relationship. The ghosts of an eight-year-old boy who existed within the man she loved.
It wasn't Diana's fault that Bruce was afraid to love her. Afraid she'd leave him like his parents had. Her immortality couldn't even alleviate his fears. But she was certain her longsuffering stance would, persuading the ghosts she was here to stay--gaining the little boy's trust so he'd love again. And loving the man who she knew loved her, even though he'd never said it with words.
No, it wasn't her fault, but it wasn't his, either.