I'm at a loss for helpful words right now but wanted to second what others have said - what you are feeling is so relatable to me and I think it is healthy to be feeling that anger as part of your grieving. It sounds to me like you are actually doing really well at detaching and setting boundaries for yourself in relation to her.
I'm about to have my first child and my AM is on a downward spiral herself. I am doing a lot of work on detaching but it's still awkward and painful fielding questions about how she's doing or responding to the exclamations of "your mom must be so excited about your pregnancy and having a new grandson!" I have had many of the same thoughts, of how infuriating it is to watch someone slowly killing themselves, at such a special time in your life when you wish they could be a joyful participant. And how painful it is to feel the void where their (emotional) presence is missing. Grieving for their potential.
I've found it helpful to read whatever I can get my hands on, including downloading a couple books from Amazon Kindle, a lot of re-reading the stickies and following the threads here and in the Friends & Family forum, and just feeling the support of the community of people here on SR.