I think someone already mentioned it, but take a break if guilt is playing a role.
And I think you said sex has stopped (I didn't go back and read the topic), but you could try perhaps pulling a move on her? Or if it's gotten to the point already where that wouldn't do much then maybe communicate about it with her? My ex wife and I would openly communicate certain things and we have kids. But never been in a situation like yours. I'd try communicating with her if you still wanted sex from her and find her sexually appealing. No idea how old she is but perhaps could be a mental thing or menopause. I know it happens to quite a few guys where sex down the road slows down or stops altogether. Could he hormone issues too.
I've bumped into quite a few religious folks who are super conversative sexually. Not sure if religion is playing a role eventually because people change as they age. Just tossing it out there.