Its post-canon where Kagome returns to her own time after everything but finds she can't age. The story starts with her living in a cabin in the wilderness after basically living hidden in the shrine untill her grandfather and mother die. She runs into Sesshomaru as a normal sized dog and takes him home not relizing who he is. He reveals himself and after some talk about the past and how there are either no or almost no other yokai left they decide to stay together because they are both lonely.
I remember this one really well but no matter what keywords I use to search for it I can't find it.

I think it is this one.

Lonely places in the wild by Kanna37
This story is rated K

Is it this?
Lonely Places In The Wild by Kanna37
www.dokuga.com/fanfiction/story/7926/1

Thank you! That's the one!