The apology very nearly set Katniss off again, but it made her equal parts horribly sad and angry-- with herself, not with him-- that in the midst of the tug of war between the two of emotions, nothing actually happened on the surface. Except that she clutched his hand, more tightly than she meant to, and refused to let go.
"Don't say that," she said, agonized by it. "I'm sorry." And then she faltered, the next words sticking in her throat. She could hear his heartbeat and his breathing, sounds she'd gotten familiar with, that often lulled her into sleep, and she suddenly imagined putting her head to his chest and hearing nothing. She still remembered doing the same to Peeta in the arena.
And then she realized she'd gone too long without explaining her apology. She'd pressed too close, unconsciously, to reassure herself that he was alive, even though that was only a small comfort. Because what if he went home? Would she remember any of this and be able to do something to stop him being killed? Or would she-- had she-- actually done something, to protect Peeta or Gale, that had ended up sacrificing him (and possibly Boggs as well)? Maybe it was directly a result of her actions, not just because she'd inspired him to fight.
She really should pull away, but she was too selfish, too absorbed in her own need for the comfort and reassurance, to be able to do it. Even if it was probably better for him if she did. If anything, she was clutching his hand even tighter than she had been a few moments before.