Yay! Congratulations on your freedom...
I know you want to help her, but I think you need to understand your limitations. You cannot "make her see" how she is treating other people if she is not willing to see for herself. Her letter, full of excuses and rationalizations, was proof enough that she is not at a point to see herself as she truly is.
You're really not in a position to help her right now. After what she's done to you, your personality and strength, you're barely in a position to help yourself.
Spend at least a year taking care of yourself, boosting your self-esteem and showing yourself how awesome and valuable you are as a person. Then you'll be in a stronger position to help her, and without the vulnerability you feel now.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker