BA, my ex was a Narcissist - like your H, iirc. I can tell you how he felt about his affair - that is, the one I left him over, not the smaller ones before that
He was so full of himself, he genuinely believed I would agree to share him - and that OW would, as well, despite the fact that I was pregnant. I don't know if yours is like mine: it's true that these Narcs follow a startlingly similar pattern, but obviously there are individualities within that.
He loved me. When asked, he said he loved me because I loved him. I had to really nag to get him to acknowledge any of my positive qualities, though he said I was docile - he meant obedient. We'd been together 12 years, since we were students, and had 'made it' by all appearances. We had a stylish, exciting life together and separately.
I was the only person, in our new lives, who'd known him since our struggling days: we shared the same 12-year-old in-jokes, mutual friends back home, and the understanding that comes from shared ideals & ambitions. Also, being "docile", I'd taken on all of his obsessions from arcane elctro-funk, through art & architecture, to football and Nazi paraphernalia
Nobody knew him like I did. And, since we shared all his interests and jokes, I was ever so much like him so he knew "me". I didn't really know myself at all. We were immensely good friends, partners and lovers - though I'd been faking the sex for a few years (it was all about him, natch).
In Jo, the OW, he thought he'd found a similarly docile, adoring replacement. She was his PA [rolleyes] so appeared to look up to him: worshipping his every move, as it were. She rapidly learned all the in-jokes and added a few of her own. She was thinner than me, so wore the mandatory designer gear better than frazzled, pregnant old me. She was also kinkier than me sexually. Alan felt that - while I was his 'soulmate' (as if he had a soul, haha) - she enhanced his life in terms of sensual pleasures, appearance and newness. It wasn't about Jo being better than me, it was more a case of him doubling his adoring fan club.
If you're interested, she stopped hanging off his every word after two years, so he booted her out. Obviously he thought I'd want him back then 