'nette, I'm sure others will be along soon who can put things into words much better than I can. I can't even imagine how hard this all has been for you. She's still your daughter and I know that you love her and are still very concerned for her. I know you're looking for answers and trying to make sense of it all. But, at this point, trying to make sense of her actions and trying to understand why she has made the choices that she has is doing nothing but torturing you. There ARE no good answers for why she does what she does - she may not even know herself.
There are many, many good threads on "detaching" on this board and you may want to search some of them out and read them. Doesn't mean that you don't still love her and care what happens to her. It means accepting the fact that she is a grown woman and that her choices are her own, as are the consequences. It means that you don't allow yourself to get so involved and entwined with her problems and identify with her so closely that she brings you down with her. She is an adult and you have absolutely no control over her actions - only she does. I'm sure you were a wonderful mother to her, but she made her own choices and picked her own path and you are in no way to blame for anything that has happened, not the tiniest little bit!
You have been a real God-send to the grandchildren that you are raising and hopefully also to little Isiah before too long. It's a wonderful thing that you are doing and I will continue to keep him in my prayers that he heals quickly and completely. Sending lots of hugs.