Changing someone altogether is changing what they have been conditioned through years of life to become. Unless she herself wants to change, there is nothing that can be done. One can change someone's mind about a certain idea or subject through magick by means of influencement to help them to see the better choice or the one most beneficial to you, but altering a person's ways of behavior entirely is something only a therapist or the person themselves can accomplish through desiring a change.
I went through the same thing with my mother. And I tried various ways to have her see her ways and make a change. She was a very sweet, loving woman, but her relationship with my step father, who was abusive mentally emotionally and physically made her slowly become a very ugly person. I tried to get her to leave him, but she was too mentally torn to feel that she could live without him, and anything magickal I could have done, would have probably driven her insane due to the mental tight rope she was walking. To me my mother "died" because who she became was not who I knew her as. I don't speak to her anymore due to the fact that this consumed her and I blame her life choices to be with my stepfather for now over 15 years as the cause. Some people can't be helped, and they will drag you down with them if you let them. The lyrics by Papa Roach "Tear My Heart Open" rings very true.
The only thing you can do is speak with her and try to get her to see how she is acting and how it is unhealthy. There is probably some factor in her past or present life that has made her so negative.
Also try to get her to see that it is causing you to be upset. Sometimes people won't make a change for themselves, but will do so for someone they love. A sweetening spell may help facilitate the conversation.
But if there is a factor in her life that has caused her to be this way and it is still apart of her life or she refuses to see that her ways are unhealthy, then its best for you to keep your distance and move on. Learn from her mistakes in life and do not repeat them.
If she agrees to try and change, the best course of action is a healing spell to help the process run smoother
Blessings, and my sympathies for having to go through such an experience.