Bear in mind I am in my 7th decade, so these books may not have been in print after the 1970s.
I believe it was a trilogy. The start of the series is a young girl who has been orphaned who moves in with relatives. Her parents were horsepeople, the relatives are not. She mourns for horses as much as/or for part of her mourning for her parents.
She starts spying on a horse owning neighbor, hiding in the woods above his riding ring. One day, horse owning neighbor (HON) gets a frantic call from the house about a long distance or overseas call; HON is conflicted, because he has just gotten on/started to warm up. Herione runs down the hill and offers to hold his horse. Having no other choice, HON agrees. When he returns from his call, he finds his horse with stirrups run up correctly, girth loosened and being walked, correctly and competently, by the herione.
Neighbor’s barn becomes herione’s refuge and solace. Herione works for neighbor, neighbor takes her hunting. Barn becomes her family.
Other details I remember: for Halloween one year, she dresses as Jorrocks. Her parents apparently read her Surtees.
At the end of the series she is an older teen, and makes a stunning entrance at a party after cutting off her pigtails in a chic short bob and wearing an emerald green dress.
If anyone can identify these books, let alone tell me where to find them, I will be forever grateful. Trying to remember details of these books (and clearly seeing the illustrations) has been a feature of Insomnia Jeopardy lately.
Thanks in advance!