She is convinced that her heart is stronger now because of Bill. Even when she first arrived at Malory Towers, she was still weaker than the other girls, a delicate little thing that some of them were afraid to touch, as though she would break. Bill has never been afraid – gentle, always, but never afraid.
But it has grown stronger now, and doesn't bother her nearly as much as it used to. She almost feels as though she could leap about the place the way the other girls do, even though she'd much rather take out Merrylegs and gallop around.
She has thought about it, and the reason seems obvious. Before Bill, she had no one to love. Oh, she had her parents and her governess and her relatives and her horse and her dolls, but that was an ordinary kind of love, the sort of love that was expected of her. It didn't frighten her with its intensity, nor did it consume her. Her heart hadn't needed to be strong.
But now she has Bill in her life, and in order to love her completely, the way Bill should be loved, the way she does love Bill, her heart has been forced to strengthen, to accommodate all of this love inside her. To give it out, and to receive it, and she is eternally grateful for both.