Half-Hearted Love
Tanjirou is not lacking, but rather too much. And still, Tanjirou deserved far more, deserved better than anything Shinobu could give. Because it isn't true she had none to offer. She does. How else could she have accepted Tomioka's heart then if she had none to give back?
But you broke his heart. That didn't make her feelings any less real. She didn't regret Tomioka, but she was ashamed of how half-hearted she'd been with him, and even more with how they fell apart.
Tomioka never deserved that. And Shinobu is terrified of repeating history if she returns Tanjirou's affections.