Wow, the thing I was most shocked about in your last post is that you were surprised when you talked on Friday about dates and times. You still don't actually know how many times they met and had sex and you are 2 years on from the affair. No wonder you've been having trouble getting past this, Christian 
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Right, so let's have a recap here. At some point long before the affair started and he had tested the water with that text joke (I'm assuming it was a innuendo-laden one) your H and this woman engaged in a process of "mirroring" about their likes and dislikes. This (probably entirely false) discovery that they liked the same things is a well known process that affair partners go through on the run-up to an affair. Meanwhile you are "dragging up the rear" and feeling excluded, probably because you can't pretend to like Coldplay, Spooks or admit a previously unknown desire to visit China 
. This was a process of "grooming" - make no mistake.
How did I guess that the OW was at a Christmas event with you? How did I guess that you felt bad that Christmas and put it down to you always feeling a "bit flat" in the interval between Christmas and New Year?
You also say that you were miserable about life on those walks with the dogs and that this meant you let them effervesce about their amazing shared interests. What were you miserable about and with hindsight, were there ever times when you were surprised by just how low you were feeling, wondering whether your misery was disproportionate?
This is what happens during the pre-affair period. The partner who is imminently going to be betrayed starts to feel unsettled, but because their trust default is enormous, infidelity is the last thing on their mind. So they pin their misery on other things, but might briefly wonder why they are feeling this bad.
At last we have established that he was grumpy and difficult before the affair. He stopped giving to not just you and your relationship, but to the DCs too. I bet this got much, much worse when the affair "went live", right up until discovery.
Have a look at the following chronology of an affair and the effect on the betrayer's behaviour:
Initial friendship/mirroring stage: No danger is perceived and the friendship feels "safe". Mood might be even better at home, since the spouse has an extra "spring in their step" as they start to look forward to interactions with the affair partner. Just before the end of this phase, spouse starts to notice more faults in their partner. If they are feeling down, upset or miserable, whereas once there would have been concern and care, now there is irritation. He will start showing affection at inappropriate moments, coming up behind her when she is putting something in the oven or at the sink. When she doesn't respond in kind, he will sometimes verbalise that "most people would be glad their husband of 20 years still wants them" while internalising "Hmmm...my female friend wouldn't do that..."
Meanwhile female friend is gushing about how much she misses sex, how she has always loved it, how she has done up her bedroom in the hope of it seeing some action, how she has never been known to say "no". We all know the script, don't we?
Pre-affair permission-giving stage At the start of this, there is still no conscious acknowledgement that an affair is on the cards, or that an opportunity will be accepted. But sub-consciously, the spouse realises that he needs to reduce the marital connection to create a gap that will allow him to say "yes" when the time comes.
So he stops doing things that will keep the connection going. He will botch things up, forget tasks that are important to her, stop doing things that make her life easier. If she remonstrates (even kindly), he will go on the counter-attack and accuse her of "nagging". She will think "How come I'm the bad guy here?" A disproportionately bad argument has come out of nowhere and about something relatively trivial.
He will restrict affection to just before sex, knowing that his partner will refuse sex because she feels unloved. This situation is a complete manipulation.
Having been selfish, grumpy and lazy all day, he will wait until she is almost asleep and ask for sex. When she says no, he will chalk it up as another point against her.
He will set tests that he knows she can't or won't comply with. A surprise weekend without the DCs, but so imminent that there is no childcare sorted or arrangements made. A night out to somewhere she won't want to go. The trip to Relate may or may not fit in here.
He might start to say things in company that jar and hurt a little.
On and on it goes, until the connection between the couple is reduced and it is therefore much easier to give himself permission to say yes to the affair partner's agenda, which is now becoming clear. He still won't admit that this is his agenda too.
Affair Stage Behaviour at home is now markedly worse. Arguments happen not just in response to complaints of the unsuspecting spouse, but because of complaints by the unfaithful party. He finds fault with everything and everyone. He becomes openly critical and horrible to be around. He appears to be disproportionately stressed and angry. The spouse reacts with disbelief, concern and more latterly, anger. Other people start to notice and the DCs suffer too. Occasionally, the unfaithful party starts having uncharacteristic arguments with other people.
Meanwhile, however bad things have got for the faithful spouse, if a man came on the scene and offered everything her spouse wasn't at that point, she would still say "No" because she wouldn't be unfaithful and can still love, in the face of horrible treatment. Instead, she blames herself to some degree.
Does this resonate at all for you Christian?