@Xeredge
While it may seem that it's sudden, it's basically a realization of everything that he put her through. He never understood his wife, he married her for political reasons and saw their marriage as just something that was necessary. It's only after spending time after her death that he began to realize just how different things are without her. And then reading her journal, it's only then he realized just what he had done to her.
Reading the journal, questioning her actions, he began to understand that because of him, because he married her, because she became the empress, he deprived her of everything that brought joy to her life. She brainwashed herself into believing that she had to become this "perfect empress".
Realizing that because of him, she basically sacrificed her own soul for the sake of being an empress, he hated himself for putting her through that.
As the saying goes. You don't truly appreciate what you have until you've lost it.