You shouldn't be talking to your ex. You should be concentrating on forgetting about her, getting her out of your life, and improving yourself physically and mentally.
You probably miss her. I can understand that. Just remember why you broke up. Chances are still she'll be the same. People never change in a short amount of time. It takes a while. And in most cases, they never change.
Why is your life her business? Your life is your own business. Even though you did a one night stand which you understand was bad, you can find a way out. Just don't talk to her. Make sure she doesn't talk to you. If you have to get new contact information so she can't talk to you.
I will do my best to reply with an educated, humble and honest answer. Ultimately, it is up to you whether you want to listen to my advice or ignore it completely. Sometimes, my advice may be wrong; occasionally, it will be right. Regardless, I want to do my best to give people answers they are seeking.