Family traditions

So for those of you don't know, and would like to know, although at this point how would you know you would like to know... ummm...where was I?
Right, for those of you who don't know, my three readers, I have cut off all contact with my mother for about a week now. All. Contact.
Now, before you think me brash and stupid, and for those of you who are muttering under your breath 'at least you have a mom, bitchypants, think of the starving children of Ethiopia', it needs to be stressed that this wasn't an easy decision to come to. It was a long, painful process, the details of which I won't bore you with just yet (I'd rather leave them for sprinklings of interest throughout future posts). Lets just say that the last straw involved the words 'Mia', 'You', 'Fuck', and '!!!!', not necessarily in that order, shouted across a busy Norwood street. Which is fine, except for the fact that my mothers behaviour towards me has been verging on abusive for a number of months now, and that if Paul or anyone else in my life had behaved in that way towards me I would have cut them off ages ago.
So.
It's been a week, and I am still at a loss as to what I should say to her. I can't tell her the truth, because if she knew how I felt about our relationship it may destroy her, or whatever happens to be left of her at this point in time. But if I say nothing, she will carry on assuming that I am the problem, that this is all about me. I can't tell you how afraid I am of having to take care of my mother in five or ten or fifteen years time, when I have a family of my own and all sorts of responsibilities. I know that many people are forced into a situation where taking care of a loved one is placed upon them, but the fundamental difference here is that this particular loved one happens to be ungrateful and self destructive. It's not as if she is ill, not in the conventional sense anyway, and yet she behaves as if the world owes her something for all her suffering. This translates directly into me becoming a cipher for all her pain and anger, as well as the only thing in her life that keeps her going: emotionally, financially, spiritually.
Thanks!
When is it enough? When do you say, no more? At which point do you walk away from someone who is draining you every single time that you see them? And how do you forgive yourself for abandoning someone so damaged that they can't seem to function without you? Can you say the words 'co-dependant'? Can you say them five times fast?