Hmm..you might regret asking me to come onto your thread then Christian because I am going to challenge some of this stuff. I also think some of your beliefs might actually be holding you back, so I post this in the hope that it will help.
Only two people were responsible for this affair; him and the OW. That is why I asked you the question, because I wondered whether you had taken erroneous responsibility for it. However you had equal responsibility for the marriage, with your H. This is a really important distinction.
I wonder whether prior to his affair, your H was coming to you with his concerns about your marriage and giving you a chance to address it? Or perhaps he didn't realise he was feeling so unloved, until the OW started showering him with attention, because that often happens? In which case, your H had a choice at that point to come to you and tell you that he had realised his needs were not being met in the marriage. I guess he didn't do that and had an affair instead? Infidelity is all about a series of choices and as you will see from the book when you read it, no-one can prevent infidelity in another person.
I worry that you have absorbed a message that because you weren't giving enough attention, your H had an affair. I wonder whether you could ask instead how much your H was giving to you before he had an affair? He evidently wasn't giving you the opportunity to see how imperilled your marriage was, but I'll bet there were other things too.
Meanwhile you are kicking yourself wondering why you didn't realise, why you didn't guess, why you were so complacent, why you didn't meet his needs. Honestly, this is the wrong approach. You trusted him and you trusted her. You should never berate yourself for trusting, because like you say, before this happened, your trust default was enormous. If you were complacent, then I'm sure he was too.
You think it was your responsibility to notice that your H was on the verge of a stress breakdown, whereas I think that it was his responsibility to tell you how bad he was feeling. But that is another example of when he wasn't giving enough, whereas you berate yourself for him not responding to a need that hadn't even been articulated. Do you see where I'm going with this?
Now the Relate angel might well be right that your H had a need for attention and adoration, but we all do, in our romantic relationships and that is not a need peculiar to him, regardless of any childhood issues. Tell me, did you feel adored, cherished and that your needs were being met prior to his affair?
You expressed surprise that the affair didn't seem to be sex-motivated, perhaps because there is a myth that this is what affairs are about. It doesn't surprise me that it wasn't the defining part of the affair, because I suspect this was all about an ego boost that could have been achieved with sexual flattery and adoration and not necessarily, sexual activity.
I think you might reframe this somewhat when you read the book and I am constantly amazed when Relate counsellors haven't read what I think is the definitive work on infidelity (and believe me, I've read a lot of books about this subject now).
However, for now, how likely is it that your H and you could agree on the following as a summary of the affair?
You and your H had got into a habit of not adoring eachother enough; life and responsibility got in the way. You were having some sexual difficulties, but you trusted eachother implicitly, as you trusted your joint friends to support your marriage.
Your H was feeling stressed about a whole host of other issues, but he didn't give enough of himself to share with you, how bad he was feeling. He pretended that he was in better shape than he was and you are not a mind-reader.
He chose instead to share his insecurities with someone else and that person had an agenda to ruin his marriage and punish you because of her own jealousy. He put her compliments and affection before you, for a while. When he realised how much he was enjoying this flattery, he chose again to shut you out and not tell you how he was feeling. When he started this affair, he treated you badly and put his ego needs above his loyalty to you. This affair wasn't about sex, but it was about shallow and false adoration from someone whose opinion was worthless.
Both of you weren't giving to or getting enough from the marriage, but only one of you gave to or got, from someone outside of it. You were denied the chance to resolve issues, because he wasn't bringing them to you and you were being deceived.
Now, one of the ways you can affair-proof your future marriage is to understand that being an adoring and sexually attentive wife does not prevent infidelity, except your own. That's because if you are giving that much to your marriage, it follows that you won't want to damage something in which you are heavily invested. Likewise if your husband is equally adoring and sexually attentive to you, that means he's heavily invested in the relationship and will want to preserve his fidelity.
This concept takes a while to get your head around, because it flies in the face of so many infidelity myths and causes discomfort in those who are smug enough to think that as long as they are jumping through hoops and catering to their partner's every need, this won't happen to them. It is the "prevention myth".
The person least likely to be unfaithful is the one who adores, not the one who doesn't feel adored.
Now, there is much more about infidelity that you'll see from the book, about individual/lifestyle and relationship vulnerabilities, as well as attachment styles and learning to recognise the red flags in your own responses to people who wish your marriage harm and I am absolutely saying that the relationship you have built now is going to be a better deterrent to infidelity than the marriage that preceded it, but that is because you are both giving.
Finally, trust is absolutely restored once you both accept that he understands that he controls his own fidelity and that no-one else can. If you know he understands that and wouldn't do this to himself again (it is under-estimated how much damage it causes to the unfaithful party) and that you cannot prevent it, trust becomes 100% easier.
You see, there will be times ahead in your marriage when your individual ego needs will be subsumed by life and other challenges. When one of you cannot give as much to the marriage, because someone or something else is consuming your attention. Knowing that your partner understands this and cuts you some slack, instead of reaching for the empty calories of an OW, will increase your security and faith.