Thank you all for your replies.
When I first discovered this the OW actually messaged me and we had lengthy conversations. I asked her the same questions that I'd asked DH and she gave me the same answers ie they had never met in RL, it was more a friendship with some sexting, but more just talking about their lives. She told me that my DH was struggling to deal with my illness!!! and she was unhappy in her marriage and would talk about that.
She begged me not to tell her husband as she said he would not be able to cope with it. She said she loved her husband and never had any intention of leaving him.
She does have form for this type of thing though, having done it at least once before.
My gut feeling is that I do believe DH when he says that they never met but I am scared that I'm just being taken for a fool.
I've asked him over and over for details and some he answers other times he says that he can't remember. It's at these times that I think there is more to it than he is telling me but I'm just not sure.
He promises that he never meant for it to happen - in his eyes it started innocently as a friendship but then along the way sexts and pictures were sent (he says she instigated this). He says he needed someone to talk to.
I just don't know. What he doesn't understand is that I find the EA the biggest betrayal. He talked to her about our life, about our marriage, about my health and my illness. This feels like such a violation. Because she was friends with him on facebook she saw photos of our children, of us on holiday. She liked a picture of our daughter in her prom dress. Apparently she talked to him about what I looked like etc. I HATE this. I think I would rather he'd had a one night stand with someone.
Is it self destructive that I want to know everything? I feel like they have had a life for 4 years, albeit a fantasy life, but I can't deal with that. I feel that to be able to move forward I have to be a part of that life. I feel like he can't have secrets with her if he wants to move forward with me. Does that make sense or am I deluded?
When I ask him he will answer to an extent but then claims he can't remember or says he doesn't want it dragged up again. That he wants to forget about. Well good for him but I can't forget about it.