Looking for the name of a book about a girl with diabetes and has a horse.?

I read a book when I was 12 about a girl who had diabetes and took insulin shots. She had a pet horse and rode alot. She was really tired of her diabetes and kept trying to give herself shots in the same places even though they were calloused over because it was less painful in those areas. I remember that she went riding on day for too long and passed out on her horse and was going in diabetic shock. I think the story went that her horse either got her home or went for help. I would have read this back in about 1978 or 79. Was a really good story.