Okay - you really do need to understand something crucial - this has got nothing to do with you being an unloveable person. You did not "drive him to it". With the type of affair your H was engaged in, this has got nothing to do with you - or even the OW to an extent - but it really is about him.
If you tell me more about your marriage and especially the time on the lead-up to this "friendship" I will be able to advise you more.
The "why" in your case could be fairly simple - and hugely dependent on your H's character and what he believed about secrets in a marriage, low-level deceit and lies - and critically, his own self-esteem at the point of starting this friendship.
How old is he and how was he feeling about himself before he entered this relationship, even at the "just friends" stage?
Assuming he is at least in his late thirties, there are some reckonings to be done by people at this stage of their lives. Many people feel that they have exhausted their potential - and if they are in long relationships, however much they love their spouse and family, there is a feeling that all the adventures in life are over. The teenagers in the house don't seem to need him as much as they did when they were little, adding to the feelings of semi-redundancy. If you were also a competent, self-assured individual, even you don't seem to need him as much.
And so along comes someone who seems to "get him" - she might also appear needy and confides in him about her unhappy marriage. This appeals to his "rescuer" tendencies and he feels somewhat rejuvenated and galvanised about life again. He has a role and someone is hanging on his every word. The "old stories" that you are so familiar with are new and fresh to this person, who constantly tells him how insightful and wise he is. She even appears to share common interests with him (although often Cat, these are completely feigned interests).
He has a spring in his step going into work. He equates infidelity with sex, so this friendship doesn't trouble him in the least. He may even appear more relaxed and happy at home at this stage.
The she starts to tell him how physically attractive he is. It feels good to be desired after all this time. This is heady and intoxicating. A polite man, he returns the compliments. He doesn't even realise that a line has been crossed, but it has.
They start to communicate outside of work, texting and phoning. At this point, he realises that he needs to be secretive and starts to switch his phone onto silent. He finds excuses to snatch moments to reply to her texts.
The text communication allows them to be far braver about their feelings than face-to-face communication allows. Eventually, another barrier is broken. This usually takes the form of a kiss, an acceptance of mutual feelings or after all this time, a date when they will have sex.
By now, he is completely addicted to the feelings this is giving him - the buzz is intoxicating and too powerful to resist. He keeps telling himself that he won't let this get out of hand, but does nothing to slow its momentum.
Now, if you've managed to timeline this, tell me how he was behaving with you at this pre-assignation stage. Was he distant, did he stop being routinely kind to you? Did he over-react when you expressed dissatisfaction with him? This often happens at this stage, because he is creating a gap between you to allow the affair partner in. He cannot justify going ahead with his plans while things are close and loving between you, so he reduces your connection.
He gets the most enormous thrill out of planning with her what's going to happen at the assignation. She talks a good job and so does he.
The awful guilt will only start to come out when the assignation draws near and realistically he feels he cannot withdraw from it. He doesn't want to either. What was he like then, on that holiday? You said he came home early.
It happens. It often doesn't live up to expectations at all. Instinctively, he knows it was a mistake. But he cannot delude himself any longer and now knows he's been unfaithful. He feels wretched. The OW on the other hand will be enthusing about the encounter and is eager to do it again.
He doesn't want to anger her by saying he really doesn't - and wonders how the hell he's going to be able to let her down gently. He feels guilt for her feelings too. So he reassures her that yes, it was wonderful etc. and perhaps agrees that it will happen again, or says it cannot but in such a wishy-washy fashion that she sees this as a chink. She thinks he is persuadeable.
This did make him see how much he loves you though - and at some point he realised that what was once an adventure has turned into a nightmare. He has dreams about the whole thing coming out. There is now a huge secret between you and it drives a wedge. This may or may not have led to him being argumentative and stressed in the early days after the encounter, but perhaps when it seemed as though this was going to be contained after all, he felt able to "come back to you".
You absolutely need to know what was happening in the contact between them in those 18 months - and whether it waxed and waned. It is probable that on some days, he thought that he could be a bit more risque with her and flirty - and maybe he was even considering a repeat experience at some stage, because the memory of the previous encounter had started to dim and he perhaps remembered it with rose coloured glasses. You need honesty about this, really you do. It is very unlikely that he has been the horrified and guilty husband throughout all his interactions with her.
Right, is the OW saying that it did only happen the once? If so, it did. The other stuff she is saying is merely designed to hurt you. If there had been no second time in those 18 months, there would not have been one after this long gap. However, it is perfectly possible that your husband was allowing her to think there would be another time, for a complex set of reasons.
He needs to come clean about this with you. He did not keep up this contact with her purely to let her down gently and keep her "on side" and out of consideration of her feelings. He was also getting something from the continued contact, the ongoing adoration and desire and he needs to admit this once and for all to himself.
The thing that will bring you enormous closure is for him to sit her down and really tell her how it is. She's probably in a huge pit of denial herself and is telling herself that he's only with you for the children/the lifestyle etc. and she needs some painful but necessary honesty. Is that possible? If not face to face, can he write her a letter or send an E mail that you can see. Since her H knows, meeting her might be out of the question - and if he is a polite but useless at emotional honesty/confontation sort of man, you may have lingering doubts about the message he will convey at this time. Give some thought to this and strike while this is still fresh. I cannot tell you how much closure this will give you.
I suspect the truth here is that this woman was never an alternative to you and he always knew that. He got in too deep and it felt like a road not travelled; an adventure and a challenge. Fortunately he was able to keep a check on his true feelings, but in order to keep that adoration coming his way, he gave it back in equal measure.
He needs to tell her just that. That any declarations of love were polite lies and that he is staying with you because he loves you and always has. That he was a fool to get involved in this adventure and he fully accepts his role in it. That he will spend his lifetime making it up to you. He also needs to tell her what he loves about you. He might never have disparaged you or your marriage to her, but I bet he wasn't effusive about you or your relationship either - again, out of consideration for her feelings and because it wouldn't suit his cause. So he needs to represent you and your marriage much more accurately now. He cannot allow her to sneer at you in this way. There can be no room for doubt in her mind and she will only believe him - she will not believe you for one minute.
His message to the OW will produce the most enormous healing Catwalker. This represents him protecting you and showing her you are a team. There is a wall around you that she - and no-one else - can ever penetrate. It is a one-time opportunity to put her straight once and for all - and actually this level of emotional honesty will bring her closure too. So many OW delude themselves about why men don't leave - and they never really move on. Given what you've said about her, I very much doubt she is giving her H anything like the level of honesty required and that is his misfortune.
This message to her needs to happen now and without further delay. Your recovery will be greatly enhanced by it, I promise you. Once that's done, it will still not be an easy road to travel, but I can help you with the various stages.
Have a think and don't disappear.