When you write that your question is "not about libido", I'm not so sure. I believe libido is strongly related to "excitement about future plans, ambition, or simply enjoying everyday conversations and activities..." Among men, I think testosterone is about excitement of all kinds, sexual and otherwise, so those things are hard to separate. But separate them we must, somehow, in order to lead happy lives. In my own experience (short so far, but I can feel things coming) I try to put more effort into exercise, activities with friends, reading novels and nonfiction that are positive in outlook, and never-ending home projects (admittedly harder to get excited about, but there's always satisfaction in "gettin 'er done").

That's kind of a "solo" answer, seemingly not directly related to your query about relationships. But it is tightly related in the sense that those are things I have to work on for myself in order to be tolerable (at least) to my loving and supportive wife of 45 years. She knows and supports this work I have to do, and I try to give back as much as I can. That's really the main project here.

It's always work; as you write, things are often "off-kilter". Maybe think of it more as "new kilter" and trying to find some reward from this journey. It sounds to me like you're lucky to have each other, and you're going to be okay.