I can't get over that I really hurt my ex girlfriend.
Hurting your ex seems to be bugging you, do you know why?
Yes, during our relationship I've been going back and forth.
Your actions have clearly had an impact on you and you aren't comfortable with how things ended. Reconciliation might not be an option, but an apology is. If you're dwelling on this then there's a good chance she is, too. Apologize, admit what you did wrong, and move on.
That's not an option, I can't do that.
Just apologize and ask for forgiveness. It will help you both get closure and move on and find better partners.