Asking Eric: Can I walk away from a foster child?

Dear Eric: Twelve years ago, I was a foster parent for a 12-year-old. “Angel” was reunified with her father after a year and when that blew up, she came back to live with me. Angel has demonstrated that she is either unwilling or not capable of working toward a healthier connection. Am I OK with being one more adult who lets her down (in her eyes, anyway) and walks away?