Some people cannot conceive of the idea of three people working together to build a relationship that works equally well for each of them. Some people cannot accept the idea of anything other than a monogamous relationship to be something that ever works
I can. Absolutely. I've seen PA & open relationships work.
But whatever you claim - they are married, you are not, they have a shared history, shared children, shared house, shared life. You are the current project in an open relationship. An outsider, and add-on. No matter how charming, welcoming and friendly they are, this is the practical reality.
I can't say I ever thought the wife was a poor sap, she seemed to know exactly what she was doing - indeed I inferred they'd done this or similar before, which turned out to be the case. I've always thought that you were the one who was deceiving yourself & said so.
At some point, when they move on to a new person/people of interest, you will realise that the posters who warned you were not trying to hurt your feelings, simply trying to alert you to your self-deception.
You have so much more to lose than they do. They will always have each other. You are risking everything emotionally, they're risking nothing. When it comes to the crunch, you're injured and need help, he offers to return, knowing full well you'll never ask him to. As long as your mum's alive and can step in, fine. But when she's not?
I totally understand after a damaging and somewhat abusive marriage that you want to avoid a full time relationship and a part time one suits very well. But a part time relationship should be on equal terms. This, whatever you say to convince yourself, is not. The fallout risks damaging you more perhaps even than your marriage.