You say you think he was mulling it over for some time but it really didn't seem like it. Nothing in his behaviour had changed and he had even made a 'snagging list' of things to do to his house ready for sale.
You're doing it again, lovely. Yes, it seemed like it to you, but it wasn't to him. And hiding his doubts is part and parcel of 'what they do'. He doubted, but he didn't want to cut off his nose to spite his face, just in case he decided his doubts were unfounded. But he didn't decide that. He decided that a ready made family was simply not for him. It's painful I know, but you need to accept that the made the choice that was right for him, even if it feels wrong to you.
Yeah but surely a relationship and feelings shouldn't be so fragile. If you properly love and care for someone then you should be willing to do what it takes to make things work
You need to consider this truth; you can love someone to distraction, but that doesn't mean they are right for you. And no, loving someone to distraction doesn't mean you 'do what it takes' if what it takes isn't what you want. In essence, this would be asking someone to go against their nature, against their own desires for their life. The result of this would be that they are unfulfilled in some way, or are very unhappy in their life. In a 'properly' loving and caring relationship, this often means letting someone go if they decide what you have to offer isn't what they need or want. In this way, yes, a relationship can be fragile.
But the 'fragility' isn't someone saying 'this isn't for me I'm leaving', it's that person saying 'This isn't the life I want, but I will stay to make the other person happy'. That really is building a house on shifting sands. And the other person won't know that until the house crumbles into the sea. It sounds as if that's what's just happened to you.
I've been on both sides of this situation. I know the pain of being the one left. But I also know the pain of staying somewhere I really didn't want to be, of 'forcing' myself to try to be happy. Believe me, it's better to be honest and leave, and free the other person to find someone with whom they can truly be happy.