Hi, and I too am sorry you're going through all this. This scenario sounds very familiar to me too. When I was in highschool, my best friend became schizophrenic and was hearing messages through the radio and began attacking her mom. Since we're not psychiatrists, we certainly can't diagnose her, but if she is schizophrenic, that too can be very successfully treated with medication. A diagnosis will probably bring you much relief.
I'm sort of in disagreement with some of the others though about not giving her food anymore. Twenty is still very young and it doesn't sound like she's in her right mind to have any clue about what is best for herself, let alone be able to work to pay for her own necessities. If she is truly psychotic, she is cut off from reality as you and I know it and really needs to be in a hospital.
I live in Missouri, and this summer I had my sister committed to the state mental hospital. It was extremely easy. I just signed an affadavit saying she had threatened suicide. If she is attacking you, I can't imagine she wouldn't be considered a danger to others. I think it's ridiculous that this varies between states. Our medical system is so messed up in this country!
I know it is soooo hard when you live with someone who is mentally ill and they say things that are hurtful. I attended Al Anon for a few years because I am in a relationship with a recovering alcoholic. When he was diagnosed with bipolar disorder, I felt almost betrayed by Al Anon because I felt like it couldn't help me with the mental illness part. Now I see there isn't much difference between the alcoholism and the mental illness in terms of Al Anon principles and the steps. I am powerless over his illness. For some reason, that was SOOO much harder to come to terms with than being powerless over alcohol. But, finally I was able to do it.
I would also suggest (although I know that this is easier said than done) that you try not to take the abuse from your daughter personally. It's the disease talking, not her. This is something else I have been trying to do, although it is hard, because my heart breaks everytime my boyfriend becomes irritable and mean. Luckily, nowadays, this only happens when he forgets to take his meds, and I can say to him "you're manic...you need to go in the other room or for a walk" and he can calm himself down a bit.
I wish you the best of luck. I think things will get better for you once she gets a diagnosis and the proper medication. I will say a prayer for you.