I'll add my voice to those expressing sympathy, Floss.
I thought you might like to know that you aren't the only one who goes through this. It might be some, small comfort, I don't know. I have two close friends who have been in similar situations and know how terrible it is for them.
The first is considered useless by her mother, who nags, is unpleasant, b*tches and says similar things to what you have related. She avoids going home at all costs, and copes by saying nothing to her mother in reponse. I've met the mother is question, she is just a nightmare. I had to leave the room before I said something to her, myself!
The other friend was adopted. The moment she turned 18 her parents announced they hated each other, had only been staying together to give her a family till she came of age, and that they didn't want anything to do with each other or her anymore. I can only imagine how she felt being abandoned a second time. She has now, mostly, managed to reconcile herself with them, and sees them regularly, which is something of a blessing, though I don't think she's ever quite recovered from the shock.
Whatever happens, don't loose your self respect. Regardless of how heartless anyone might tell you are being, you have to keep faith in yourself and continue to believe you are in the right, here. (Even if you aren't.

) You certainly have my confidence and respect, however feeble and useless it is in comparison to that which you really want from your family.