"There is a belief that a relationship has a value in and of itself - not because it is good and brings happiness, but just because it exists. And that the relationship should be sustained at great personal cost because it is more important than you are."
Yes, yes, that's what I've been trying to say for a while but couldn't quite express it right to myself - that's exactly it. Just because it is called a relationship does not mean it has to carry on being one even if it's bad. Just because you're with someone doesn't mean you have to carry on being with them whatever they throw at you. Of course nobody's perfect, and making a home with someone else involves some sacrifices and some adaptation. But that's about two people, with shared values and objectives, juggling about a bit to find a comfortable fit between them. It's a beautiful thing when it works. When a comfortable fit can't be found, or when one person's doing all the juggling while the other takes all the comfort, that is not a relationship worth maintaining.
When XH accused me of "breaking up the family", I tried to say that a family is not an entity of itself, it is the sum of the people within it. If the happiness of the majority can only be continued at the expense of one member's misery, that's not family - that's slavery. (And believe me I am completely satisfied that I did everything that could reasonably, and even unreasonably, be done to keep that marriage going. I called time when I realised my mental health was being badly affected, not, as XH tells people, because I was bored and looking for excitement with younger men. I wish I hadn't been so stubborn trying to keep it going, as at least one of the DCs is still suffering from emotional fallout, but at the time I thought it was just me. How we delude ourselves.)