Abitwobbly... I can see why the term 'mistake' would grate, in the same way that people who start smoking would use it - as would women who pass off another man's child as their partner's - or those who hit their partner or child as a one-off. All of these things - and many more - are coined as 'mistakes'.
From your question, I would say that it's possible for a 'mistake' to happen in that an unexpected kiss can take you unawares. What your next move is from that is what dictates what happens next. A sensible and cognisant person would say something like, "Oh, that's not what I want to be doing, sorry" - and not allow themselves to be in that position again. I think if it has got beyond that stage, it's not really a 'mistake' in terms of not seeing the potential for harm. Make a choice to have sexual intimacy with somebody is a conscious thing and I don't really think that anybody gets that carried away with themselves as adults not to realise that.
The point I was making is that, unless you have been in this position yourself, you can't look inside another person's motivations and decisions for what they really are. I despair of the smuggery that I read throughout this board at the moment; denigrating, dismissive and just plain nasty in many cases, with a lot of over-loud 'advice' that makes no sense.
You talk of addiction; an affair is not an addiction - not the affair itself - but the affair partner certainly can be. I don't believe that the partner of the cheater could ever be held to account for the affair - that 'blame' lies solely with the partner who cheated - but I think there are often many quite obvious cries for help/complaints about the marriage/partnership, that go unheeded and perhaps the pair will never know whether the outcome would have been different had both been more attentive to their most special relationship.
Having been in the unenviable position myself - from both sides - I will say that from my own experience, an attention deficit - or perceived/real hurt - can make somebody vulnerable to an affair. I don't know how it 'happens' as such, but I do know that when the line was there, I didn't hesitate as I believed that I could 'compartmentalise' (as your husband did/does) and keep it from my partner. Some women are just as successful at this deceit as some men are. I would say that those people will not feel guilt unless the affair is discovered and the 'hurt' rains down on those they love. I wonder if your husband is remaining 'cold' as an antidote to this? He's hurt you badly and he knows that this particular bell cannot be unrung.
Please believe me when I tell you that I'm not an advocate of affairs and that I do not believe that your husband had any intention - or wish - to hurt you. However, his actions, formed with arrogance and feeling of invincibility, have done just that. He will be feeling pain of his own - as will the other woman. Nobody escapes, not ever, even if the affair is undiscovered and real lives are resumed. It would be a wonderful thing if, at the point of line-crossing, the orange Tango man or similar, would come at the potential affairees and shove a rubber glove in their faces to bring them up short... all they get is a heart-stopping moment of inevitability, which they succomb to or, if they are very lucky, a feeling of intense dread, which repells them from taking it any further.
I'm sorry for your pain, it rings out in your post very clearly and I hope that you are done with it very soon. This may not be a consolation to you but if you can think of yourself moved on in several years, with all of this put to bed and behind you, living authentically in your new life... whilst your husband has all of his grieving to go through. It sounds odd given that he doesn't seem to be 'suffering' now as you are, but that seems to me indicative of self-preservation. Like all arrogant people who believe they can 'have it all', realisation of their 'mistakes' hits them like a steam train so they will defer that wake up call for as long as possible. It comes though, it surely does - by which time you will be safely out of reach and, I hope, happy.