Do relationships that start as emotional affairs and move in fast ever actually work? Looking for real experiences

My now ex and I were together for just over 8 years. We were in the process of buying a house when he suddenly did a complete 180 and ended things. His reasons were that we’d become “comfortable,” weren’t doing things anymore, and had grown into “two different people.”

He was adamant there was no one else involved. Unfortunately, due to circumstances, we had to continue living under the same roof straight after the breakup.

A couple of weeks later, I found out he had slept with a coworker within 24 hours of us ending things and was having an emotional affair with her in the months leading up to the breakup. During the time we were still living together, he continued “hanging out” with her while lying about where he was and who he was with. I had no idea she even existed until the truth came out. He truly believed he was going to get away with it.

About a month later, we both moved out of our shared home. He moved straight in with her and her children. He doesn’t have kids of his own and has never lived with children before. We also have a custody arrangement for our two dogs, so we’re unfortunately still in each other’s lives.

I’m struggling to process how quickly this all escalated and how little remorse he seems to have. It feels like I’ve been stabbed over and over while he gets to move on and play happy families.

So I’m genuinely asking:

Do relationships that begin as emotional affairs and then immediately turn into full-blown relationships and living together without proper dating, ever actually work long term?

For extra context, he is extremely avoidant. He struggles with responsibility, communication, and processing complex emotions.

I’m struggling to comprehend how someone can blow up a long-term, stable relationship for what feels like chaos: moving straight in with a new partner, her children, no real time spent dating, and no shared history or depth. If you’ve been through something similar on either side, what actually happened once the initial intensity wore off?