zoo, you don't have to try to explain why when you tell her that you won't be able to visit, but you will be keeping her close in thought and will call to visit with her on the phone (if that would feel okay for you to do).
You can send cards to brighten her day, and maybe tuck small things like bookmarks or photos inside.
I wonder though, if you would be recongnized and if you were, if the person remembering you would be thrilled to see you doing so well.