Convince my ex-girlfriend that we can be great friends?

I can't blame her either. But you have to prove to her that you can act civil and control your emotions. I think that she's afraid that you'll walk out on her again. If she doesn't want to be friends with you then don't push it because then she might become afraid of you. Give her time, when she is ready to become friends with you again be open. Let her know that you'll always be there for her if she needs you. That will lessen her anger towards you. :)