pain, and Belldandy was glad of that, for she knew it hurt him to see her

so.

.... it would hurt him...

There was nothing else to do, and her husband seemed to realise it.  He
closed his eyes, and settled into a deep sleep.

Actually there is much else that can be done. It seems to me that Keiichi
should say more to help Belldandy accept his mortality, you could have him
show her how to treasure her memories of him- his last legacy, instead of
leaving her to figure it out on her own.
This story left me very, very miserable. However, I have thought of a way
to balance it. I could tag it to the end of your story, with your
permission
of course.