It sounds to me like your mother is profoundly depressed. She is looking at the world through a filter of negativity and despair. There’s a saying that every cloud has a silver lining. But for your mother, if someone gives her a silver lining, she immediately puts a cloud around it. It is a sad way to live, especially since it is so advanced that she can’t even let the love of a caring family help her change it.
You didn’t mention whether your father shares your concern. I certainly hope so. As a daughter, there is very little you can do to change your mom’s attitude. But your father may be able to encourage her to get into treatment with a mental health professional.
It is not healthy for you to deal with this constantly. Since you can’t yet make a home of your own, you need to find ways to “leave” without physically leaving. Leave for school early. Get involved with school activities or a part time job that keeps you out of the house as much as possible. Find other young people who are doing positive things through volunteer work or an activity you enjoy. Spending time with them will help balance out the negativity of your mother.
Don’t argue with your mother about her attitude. You can’t change her. She will only change if she decides to get the treatment she needs. All you can do is let her know that you love her — which is a lot.
I wish you well.