Here is a question I still struggle with. My ex told me she hadn't loved me and resented me for years. How did she hide it so well, how didn't I or anyone we knew have a clue. I guess that is just one of the things that shocked the hell out of me, just being blindsided, this was shortly before I found out about her affairs.
Simple - she was surviving the way she knew how, it wasn't about you - it was about her survival.