It’s been a while since I’ve watched, too, but I remember this part because most people misunderstood her motives for cheating, and I’ve had to explain it before. Walter had shown her, at this point, how awful he was and he had also made it clear that she was stuck with him, doing whatever he wanted, whenever he wanted, and that he wouldn’t leave and she couldn’t leave. She was trapped with him. The cheating was a chess-move, a deliberate choice to make him leave, because it was the only way she could make him.