I've now read and read the most important sections of the DR book to me:

Last Resort Technique
Infidelity
Midlife Crisis

On Infidelity, p.214 talks about When He Won't End The Affair
It mentions that people will call me a doormat (nearly all do!) and to trust my instincts. That I'm in for a tough battle with pride and emotions getting in the way and that it will take courage, stamina and blind determination. My friends are quite annoyed with me not following their advice so I'm being very selective in who I tell that I'm following DB.

It says I need to work out what appeals about the OW and better fulfil that need. He's told me straight out that she makes him feel happy and relaxed. So I'm making no demands of him at home as that was a huge issue and I'm trying to be loving, in good spirits but a little detached. I'm ensuring the house is cleaner, doing more home cooking, trying yoga, going out more. He had said that he'd noticed I'm listening more and not interrupting so when he wants to talk about his work, listening actively and gently prompting him to tell me more with follow ups.

I'm also cutting back on the messages, letting him message first and only contacting him about the kids/domestic issues. However the messages and memes were a huge part of the emotional affair he had with the OW so I do send him the odd joke on Instagram which he appreciates and responds to and he sometimes does the same. Keeping up communication with no pressure.

Problem is that by following this, he seems happy and pleased to see me and wants to spend time with me watching tv etc. but this also makes him more affectionate and he's wanting to kiss and cuddle me which I know isn't recommended! I'm trying to gently enforce boundaries but I'm worried about pushing him too far away again. The book says I need to be the person my spouse wants me to be so that he might realise the grass is not greener on the other side. So I'm no longer mentioning the OW, the affair or his plans to move out.

Just taking it day by day. I know H feels very guilty and he seems to have some doubts but I've not asked how he is feeling (and nor will I). H sometimes mentions future finances or co-parenting together but is making no concrete steps towards moving out that I can see, other than transferring his salary. I don't ask what he might be doing behind the scenes though and he claims to have been paid less this month which is a huge red flag.

Ultimately though, he still holds resentment over past issues, is embarrassed that lots of people know he's left me for a younger woman (and is a cheat) and finds it easier to cut ties and move on than backtrack and fix the problems and face the judgement of others.