I like what Betsey says: her XH gave it his best effort. It was a lousy effort, but it was the best he had to give.
Claire, I know this is hard to understand, but there's some validity to what MB says here. From our viewpoint, they didn't try at all. From theirs, they did everything.
I dated a guy in college who just wasn't capable of loving anyone the way I wanted to be loved. He just couldn't. And I know he was trying, he was giving me his very best effort. I recognized it at the time. It still wasn't enough.
Maybe it's not that your H (and mine) didn't try. Maybe they just aren't capable.