I think he just never looked at her as a person, he said himself that she was like an oversized doll so maybe he really saw her that way. I’m not saying that makes his actions ok but it would make his massive 180 a little more believable. He read her diary and realized that she was a human being, and one who struggled a lot, especially for his sake. Maybe there were a lot of other things she did for him that werent mentioned, but imagine finding out someone devoted their entire life to being worthy to stand by your side, and the whole time you couldnt even look at them as a person. The guilt would be massive.