My mother is like that; non-stop complaining and nagging. Since there's not changing her, the only thing I can do is to try to control my environment and limit contact with her. I moved out at 17, almost 30 years ago, which made it ieasier, if you're still at home, my suggestion is to limit the time you spend at home, spend more time in your room etc.
Also try some basic "dog training", as in rewarding desired behaviour and ignoring undesired behaviour. (This worked for me with my father /unfortunately not with my mother). He doesn't complain, but he has a terrible temper that he used to take out on me and my mother. He has learned to control it around me, though, because I simply got up and left every time he as much as started snarling. When he's nice and pleasant, I'll stick around.