Thanks so much for your replies and advice so far. Really appreciated!
How did you find out? Did he confess, or did you catch him? IMO, that makes a big difference in whether or not you can trust what he's saying.
I found out because he was acting dreadfully to me and had stopped having sex with me when he was planning it and after the event (he even had the audacity to suggest he might have cancer as a reason he wasn't able to have sex with me). My gut 'knew' the moment I saw his face after his night away, but I only checked his phone a few weeks later when his behaviour towards me had become utterly unbearable. I confronted him immediately after finding evidence (probably not the best step) and he admitted most of the details immediately, although he minimised a number of things about the actual meet-up. He read 'How to Help...' and since then has, I genuinely think, told the truth and been patient with my questions. That book has been a real game-changer in terms of 'getting it'.
Did he say something to this effect, or are you assuming it? What he did isn't polyamory because he didn't discuss it with you and get your consent to open the marriage. He's just a plain old garden variety cheater who trolled the internet to find some strange.
I totally agree that this isn't polyamory or an open marriage - thank you for validating my interpretation of those terms, neither of which I have a problem with as a choice for other people. A few months before his meet up, he had gone on hook-up sites and even left the house to meet someone, only to get cold feet and show me his activity online voluntarily. He had talked a few times about regretting having limited sexual encounters before we were married, seeing me as having had much more experience (who cares?!). I now feel both of these things were ways of him broaching the idea of an 'open'(unilateral!) marriage in a manipulative and unhealthy way - almost like a threat. He knew my feelings about this (monogamy is what I value). At this point, he swore he would never do anything. However, just a month later, after an argument, he had 'met' the AP online and was planning the meet up. This was totally hidden from me (although his behaviour was neglectful and contemptuous). He has said that he actively tried to construct a 'just sex' bubble, and meeting a married person (I still think this was particularly immoral) was designed to keep each other confidential. Everything in his tech records accords with his tale, even though I did catch him out on some things before he read 'How to Help...'. I do think he has been 'honest' since having read 'How to Help'. Some things, which I had originally asked (such as more explicit sexual stuff), I have decided not to re-ask as I feel I don't need to know such intimate details at this stage given where I am in my healing. I have realised that, for me, such stuff won't give me the answers I actually want, which are the big 'whys' and 'what nows'.
Like you, I wondered if I could ever be enough. I wondered what the AP knew about him, and if she knew who I was. Did your H share his information with his ONS? He doesn't know her last name; does she know his? In my case, the AP was single, so I was the only one getting hurt. In your case, though, it sounds like the AP is married. I think maybe I'd ask that question on a polygraph, too. He could be lying about knowing who she is to keep you from telling the other betrayed spouse (OBS) and having to face the repercussions.
He stupidly gave his real name and details, and he has a relatively public profile online -which includes pictures of us together, which is why I feel so exposed; I know she has looked me up on LinkedIn, because I know where she works - it is a huge organisation, so not 'searchable'. I genuinely think he didn't know her real name, as he told me her name and then was visibly horrified that I might contact her/her husband. I am pretty sure that the name she used was just a username - I tried obsessively to find her in the first few weeks, but there is absolutely no trace of her. The name she used was that of a minor actress from tv. In terms of why I want/ed to find her: 'justice' and to try to convince myself I am more desirable than her (yes, I am aware that sounds pathetic). He has said repeatedly that there were absolutely no comparisons other than ones to my benefit (of course, he would say that, and the big comparison was that she was willing to do what I would never do, which is the biggest boost to my ego). However, I am trying to figure out if I should expend any more energy on trying to find her; much as I would love to punch her in the face and call her horrible things (I wouldn't actually), I feel that, again, the 'why' and 'what now' is directed at my husband, and I would not ultimately get 'answers'. If he seriously traded down, like he says, it won't make me feel better that he acted as he did. I don't know if one would ever feel 'justice' knowing who it was and having the power to potentially contact her...But perhaps some of you have found some restitution of the power balance by knowing?
Regarding the depression, I have decided I shall set some clear limits about what I will/can help with and what I won't, and what I will accept and what I won't. It is probably wrong, but I have started calling him out when I think he has been manipulative of me in this regard. He has just agreed to get medical help, which is a huge step in him taking some ownership over the bigger issues. I think the problem is that it is hard, when face with someone I still care deeply about, to see him in such anguish. I have expressed my own anger and upset, and he is open to talking about the infidelity whenever, but I still seem to be expending lots of mental time wondering how to help him.
Thanks so much for listening. Sorry that I use lots of brackets!