I think Dr Glass mentions the weird urge that many affair partners have, to get their public partner and affair partner in a room together. It'd already happened to me when I read it - and to my friends, and happened to me again later - and, you know what? You know. The social veneer constrains us to play nice, feeding our errant partner's delusions or fantasies, but we're on edge.
You weren't mistaken about the little kitchen visit, Faulk. What on earth were they supposed to have in common over an asparagus tart? Are they baking together now? Of course, there was one, because it provided cover should cover be needed. And it is appalling that he's told her a personal anecdote he'd never shared with you. She said, "Oh, tell them your New York story, Musicman!"? Who does she sound like? His girlfriend, perhaps? Somewhat proprietorial, anyway.
You can't carry on like this, surely? It tears you up. The core issue is not what they're up to. It is that he's taking effort, engagement, time and enthusiasm out of your relationship for the sake of another one. He is lying to you - taking away honesty and trust. You are unhappy; you've told him so, and why, and he immediately went to do the thing you mentioned. He is not demonstrating love, concern or respect for you.
I think you need to get your head very clear on what's changed in your relationship. I'm so sorry you're going through this. You do sound ace! It's horrid to find yourself feeling like an anxious teenager.