or that she could truly escape into a time-portal in the garden behind her house. (I can't remember the name of that book, but it was one of my very favorites.)
   Tom's Midnight Garden perhaps?
THANK YOU-- I think that really may be it!!  It's the same kind of story as in the classic movie 
Portrait of Jennie.  My absolute favorite book as a child.
Other favorites from about that same period of time include 
Gone Away Lake, and 
The House of Thirty Cats.  (About 8yo, I think.)  There aren't too many books that I can actually stand to read over and over, and never have been.  Those are among them, however.
Someone else mentioned 
A Summer to Die;  I also remember that being one of those books that I found profoundly moving, but that nobody else I know seems to have ever even heard of. 
What a 
wonderful thread this is.  
