My ex-husband told me, after our marriage was challenged by my descending into
three years of agoraphobia and c-ptsd, that it would have been better for him, if I
had had cancer. His words "I could have understood that!".
There's no stigma attached to being the partner of a person with cancer, or MS, or heart disease, etc etc...the cancer-spouse is called a 'hero' and it's, quite frankly, a more socially accepted and approved medical condition. We're not there yet with any version of a wellbeing challenge that falls under the umbrella of 'mental illness' not attracting stigma.
It's like the lyrics in that Joni Mitchell song....from the movie Love Actually
I've looked at love from both sides now
From give and take and still somehow
It's love's illusions that I recall
I really don't know love
Really don't know love at all
You and your partner could start discussing how pernicious messages of societal stigma, attitudes of family, friends, workplace colleagues and internalized stigma have accumulated over those difficult years and eroded the bond that you shared going in.
We always hear or read about couples who have gone through a rough time and invariably they say (to the camera), "it's made us stronger... our relationship is deeper...it was the making of us". So what happens when you're not that couple...but you bought into that expectation.....
})
It was really a turning point to understand that my then husband was quite happy to use my little ol' 'mental illness' as a scapegoat for what he preferred not to confront in his own personality. Used to be called "shadow work" in the 90s. 
