How do I stand up to my mom?

That really sucks. :( You can talk to me if you need to.
Well, if things really get steamy, I would suggest get out of the house. Talk to someone, whenver it´s you teacher or a total stranger (like a police officer) It´s not legal for your mother to threaten you. Spend as much time as you need outside, alone or with some friends, but try not to stay too long, you mother might get mad for having you outside the house for so long. DON´T wait till she really beats you before you go seek help, do it before something happens. And try nto to picture scenes of her dying, it can worsen the situation. Whenever those thoughts appear, just shut them off and think of something pleasant. Don´t think negative about everything. That´s how many people in (torture) prisons mostly get broken. Their capturers try to break them by making them hopeless.Don´t get hopeless, just remember that to every bad side there is a good side too. Whenver it´s your mother or the whole situation, there´always a bright side.Just think positive and don´t be afraid to cry if you need to.
Don´t forget, your mother is also going through hard times. Even though beating or threatening you is not the right way to handle stuff, she also has her problems and depressions, especially since her friend died. Cuting off comunication would frustrate her and most likely cause her to slip back into depression\agression, ect. I know it´s very hard to forgive her, but you don´t have to just forget the entire situation and pretend nothing happen! You know the old saying, "Time heals all wounds...." I wish you good luck and hopefully, you and your mother can restore your relationship. :)

You need to talk to her about your feelings. Do not yell or scream or cry. Calmy and respectfully tell her how you feel. Try to make her understand and if she still goes on with her ways, find professional help. If things still don't get better and continue to get worse, I think you should consider moving in with a friend/aunt and uncle/cousin/older sibling/gradparents. Maybe you two do need some time apart to figure things out.