Sunshine 2 - I'm not sure anyone ever gets the whole story. The problem is that that the betraying person minimises so much at the beginning, that these become learnt memories. After a while, some things are also genuinely forgotten, especially details of conversations. I've often wished I could see a transcript of every conversation or a DVD of every interaction, hellish and masochistic though that might sound, but at least then there would be no doubt or gaps to fill in.
Counting is right though, every time fresh information comes out, there is renewed pain. That's why I would always counsel anyone in our Hs' position to come clean right at the start - about everything. Having said all of that, on some issues, mine deluded himself that something meant one thing, when really it meant another, so there would always have been fresh realisations etc.
As an example of this, your H might be deluding himself that it wasn't pre-meditated and that it took him by surprise. You instinctively question that, and I think you're right to. You might suspect that there had been some heavy flirting and innuendo leading up to him inviting her along, but because you cannot prove what was going on in his head (i.e. his realisation that something could happen) he will insist that it was the furthest thing from his mind etc. The problem is, if he doesn't challenge some of these delusions and comfort blankets himself, he cannot protect himself from the next work do. It's going to take a lot for him to admit that he was fully aware of the attraction and therefore helped it along, but he needs to do it.
I want to pick up on points both you and Counting made Sunshine. You said
"We are very good at brushing things under the carpet as my dh hates confrontation/ emotional chats. Historically, I find it hard to forgive if someone had hurt me badly too." I understand that completely, but that was then in the "old relationship" and this is now in the "new one". One of the biggest breakthroughs you can make is to recognise that the behaviours of the past will not work now. My DH also hated those sorts of chats, but will now talk for hours about feelings and emotions.
Dealing with something as catastrophic as infidelity cannot be swept under the carpet with a series of brief chats before life reverts to normal. One of the reasons infidelity thrives in our society is because of poor communication between couples. This doesn't mean you shouldn't call time if a discussion is going round in circles or one or both of you is exhausted, but it is virtually impossible to deal with the fall-out from this without these "emotional" conversations.
Similarly, you may well have had trouble forgiving past hurts and slights, but infidelity (of any sort) is a big one to get past, because it goes to the heart of one's self esteem and feelings of safety.
It could be Sunshine, that the counselling reveals that your husband has always had trouble expressing his feelings and that this inability to discuss what's really important in life provided fuel to a potential affair. Emotionally retarded men, who only speak about sport, cars and work to their male friends, are especially vulnerable, I think. Along comes a fun, bubbly woman, who converses at a more emotional level - and the attraction is obvious.
The goal would be for your H to develop more emotionally intelligent male friendships, where feelings are discussed - and for him to open up to you about what really matters in life. You both need to connect on an emotional level in the future - it's an enormous barrier to future infidelity.
Try to have other conversations too that are not about the affair per se, but about your true feelings for eachother. One of our most memorable early conversations was about all the things we loved about the other - our qualities, the feelings we engendered in eachother. This is a positive conversation, that reminds you why you are there fighting for this marriage.
Counting, much of this applies to you too - I feel frustrated for you that your H is rationing the time spent discussing this - this means some of your needs are still being unmet, but the book will certainly help and I'm glad you're getting that. I know only too well what you mean about anniversaries - we are through all of those now, thank goodness and they weren't as bad as I'd feared. Still, something can catch me unawares - a reference to Summer 2008, or a piece of music from that time - and it hurts.
I've just started some solo counselling (long overdue tbh) but have only had one session so far, so too early to say how useful it might be.