JLA, sorry to hear about your suffering. It's certainly not only you who's doing something wrong, but you are the only one involved where you can go and change something.
I had and sometimes have a similiar situation with my grandmother (who educated me because my mom died early), only that my grandma has more people to spread all her negativity on (her daughter, her son, and her three granddaughters.
It helped me a lot, when I told myself that I'm a grown woman now, and that I really would like to get encouragement and acceptance from her, but I don't need it to get on with my life. I can stand on my own, and gladly so. And I found out that no matter what happens, I'd sooner start growing a third leg before I could stop loving my grandma. And that is true for her, too. All this negative talk is just a (very inadequate) way of her trying to reasure herself of love and acceptance. She does all that negative thinking and talking, because in her generation this was the only way allowed to women to acchieve something. A woman wasn't supposed to say what she wants (that would be too unfeminine), so she had to resort to complaining, and hoping that somebody would get the hint.
And you don't need to take all that negative crap from your mom. It won't really help her when she brings you down, and it surely makes you feel awful. Just hang up the telephone when she starts it, or go away. And tell her why you do that, and tell her that you do love her, and that you will be there for her, when she has a problem where you can really help.
And EssEmm is right: get somebody else to help with it, too.
Best wishes for you, I hope I made some sense, Eve