I am of a completely different situation than you, but I believe there is nothing wrong with how you're feeling. You need time and space to yourself. It's YOU first now, you're an adult and in charge of your own life. Sometimes, we might need to shut someone out that is detrimental to our health. If she is not a mother, and hasn't been, then you'll be losing nothing anyway. You don't owe her anything...that is that stupid guilt trip the children of the mentally ill carry (that I've carried)...that we are responsible and are some f*%$ed-up sacrifice for them. The truth is only they can be responsible for their life, and only us for ours.

Take care of yourself--always take care of you first.

I'm new here too, and I feel for your situation. I agree that now it's time to focus on your needs and your family. I'm going through a similar situation with my "mom"; I just posted so feel free to read all about her. I have so much hatred for her right now, and I hate to feel hate, you know what I mean?
My aunt came up to help me deal with this situation. She got me involved in Al-Anon, and with a support group for family members of the mentally ill. I am looking forward to self-discovery and to not having to worry about another human being to the extent that I sacrifice my sanity, my family and my finances.
When I was reading your post I could really identify. You are right in needing your space away from the insanity. "Codependent No More" is a great book that addresses the guilt we feel when we pull away from the person affected by drugs, alcohol, mental illness, etc, I recommend it and Al-Anon if your mother has ever had a problem with alcohol. Good luck, and follow your intuition.