Not to be rude, but sometimes we cannot "fix" someone we care about. I speak from experience. Sometimes you can lead a horse to water, but you cannot make it drink the water. Aspartame damages the brain cells along with many other parts of the body. I would just let her do what she wants and pray to God that she will ask you for some alternative guidance.
Don't kill yourself, We are the only ones that get hurt while trying to help.