How do I be friends with my ex-friend again and what should I say to her?

[Helen]

Why would you want to be friends with a "heartless person" who "never cared about you" and "treated you in a cruel way"?
It doesn't sound like the friendship was very mutual, and you brought out the worst in each other.
There's a difference between making peace and being friends again. It will be good to make peace, but it doesn't sound as though you should ever be friends. When someone is bad for you and doesn't deserve you, it's senseless to let them back into your life. By all means, message her to say goodbye. Let her know you are sorry for your part in everything that happened, that you are going away and you wish her all the best. Then move on to a new life in America.