"I was so busy loving you, I forgot to love myself."
She's never been a self confident person; so she learned to work around that. She lived, then and still now, in a state of low self-esteem and a determination to fix that one day.
But then she met him. And she fell in love, but she didn't mean to. She didn't want to. But she did, and it was a good love, but it didn't last.
She stopped loving him when she realized she never learned to love herself, but she couldn't tell him that. He had already learned to love himself, and she feared being embarrassed by showing someone she wasn't who she acted to be.
She cared for him, she really did, but she realized she had to love herself before she could fall for another.
But that's a secret she'll keep to herself.