To me that whole "I can't look myself in the mirror" sounds like either melodrama or something worse than simply cheating. She left you and basically indicated she wanted a divorce. From what I've seen on here, there isn't a whole lot of guilt about sleeping with someone. It seems to me most have a "why shouldn't I, I've already told him we were done. It's just a formality to get divorced". It sounds like dramatic self-flagellation as lead up to telling you so you won't feel the need to get on her case about it too much.
She may also have aggravating issues from her past (as my W does) that shadow her recent behaviors. My W has indicated in the past that her shame stems from what she's done to me in our M; but also indicates that she feels it stems from a host of childhood problems.
