This is a great question.

I think that the bombing and the question of "what if" it was one of them who died instead of Dusty, made Melanie lose some of her pridefulness. She was so self-righteously angry, (making Lindsay feel that what she did was worse than anything she'd ever done and Lindsay being made to feel like she was a bad person who didn't have any right to be forgiven) and had built a seemingly impenetrable wall. It took the bombing to knock it down.