Not much progress

[PeterV2]

She has told me she regrets ever starting the affair and that she never dreamed it would get so out of control

That's ^^^ a good thing. It means she has feelings of regret. I means she's not just carrying on with the A and using every strategy to justify it. But don't harp on this with her. You have to let her go through the feelings of loss of the relationship with the OM. She will grieve that loss. She may even bring it up in conversation with you. Just listen to her and acknowledge that it must be hard to go through those feelings. Whatever you do don't lash out at her when she talks about him. If you can't take it then just give her the time out T signal with your hands. If she doesn't get it it's ok to give the signal again and get up & leave the room & go for a walk.

Regardless of what happens in the next few days, focus on remaining detached. Don't let her draw you in to any arguments or heated discussions. When tempted to rebut any comments from her do the 180 and validate and detach.
I used to argue with W every time she brought up my ex and how she thought I was catering to her, but then I learned to do the 180 and told her it must have been hard to feel that way and left it at that. Certainly defused any heated clash paving the way for baby steps in the right direction.