It was the glass... It helped Rhea catch Susan at the end. It purposely didn't show Roland what he needed to see in order to save her. It was the glass that manipulated Rhea and Roland. In turn it was Rhea that manipulated Cordelia who then used PR to get the townfolk riled up and ready for the burning. The glass showed what it wanted and set the ball rolling so that Roland would experience this long in hopes of breaking him, and in turn, his quest for the Tower. It is only because of his original KaTet and his new KaTet that he is able to continue on his path. If it wasnt for them he might have just given up. I think that is an important point King tries to make, as strong as Roland is, he still needs his friends to accomplish his quest for the Tower.