Her inner arguments began with her visualization of meat as an edible object that reminded her how animals have to be shot in their heads, and then lose it, in order to be consumed by the world. She related herself to meat in that she felt that her mind would be brainwashed by society once she becomes a married woman with limited choices.
The cake she bought for Peter for Valentine's day was out of obligation rather than love. It was also her way of testing whether she and Peter were on the same sheet of music, and whether her refusal to eat was insanity or not. When he ate it, she realized that it was her who had a major problem.
The final cake, the woman-shaped cake was her way to show rebellion. She presented it as an offering to Peter the way she expected to become the sacrificial lamb of a male-centered world. When she was able to express herself in order to bring out her anger, she ended up eating it wildly herself: This was merely a way for her to say "I live and die MY way".