I think when she had her jobs, she was able to hide from him some of who she was. She kept up her looks and hygiene because she was around other people and had to deal with their judgement daily. We saw how she was on camera when he was around and how that was different than when he wasn't around, sometimes the startling change when he'd come home. Part of it is an act, part of it is how she is around all men who are not Peetz. Peetz she just treats like utter and complete trash.
I don't think that she changed. I think that when she lost the second job and was home all the time, she no longer had a reason to keep up the pretenses of being a decent person. When she was leaving the house, she could at least hide how much she was eating when he wasn't around. But, once she never left the house, it got harder and harder. Then there was her personality, her hygiene started slipping because she didn't care enough to keep it up. For some people, the person they love is enough, but for narcs, it never will be. So, he started to see who she really was. Who someone is will always come out, they can only hide it for so long. Some are better than others, but it does always come out. For her it came out and he eventually couldn't take it anymore.