I may be totally off base here, but I'm just thinking aloud. If you feel these things for her, then is it possible for you to go see her before she dies and express your gratitude? I don't know if she lives locally or long distance, but is that possible?
I say this because I didn't take the opportunity to express my appreciation to my daughter's step-mom when she was dying. I always appreciated how she welcomed my daughter into her life when my ex married her. So many step-parents resent the children of their new husbands, but Barbara didn't. She treated my daughter as if she were her own and I should have told her how much I appreciated that. I did visit her in the hospital and kissed her cheek, but I still feel badly that I didn't express how I felt.