I'm sorry you are feeling down, h4h. Just know that what you are feeling with your kids leaving is what she is feeling, too, when she doesn't have them. Will this be the life she wants in the end? I doubt it.
Karen and Puppy are right about letting her miss you. When you detach and let her see what it could be like without you, she calls and gets her "fix" and feels ok again. I remember doing this, too. If I thought my H was upset or pulling from me, I'd draw him right back in...making sure he still loved me and wasn't upset with me...and then, once I was reassurred continued on my way. If he wasn't mad at me, and was still my friend, I knew I was going to be ok. I couldn't fathom not having him in my life in the way I needed him. Does this make sense?
I still think she is getting what she needs from you, and living out her whims.