Everything said in this situation looks like retreating. She slammed you with the final offensive, and you can only go backwards from here on out as long as you keep talking. That is the key though: as long as you keep talking. It would have been wise to just give her the cold shoulder indefinitely and move on with your life. However, you did respond, and that is okay for now.
Right now you are focusing on what you could have done. The past is done and over with, you have learned your lessons here. The lessons are these:
1. Don't invest too much into a relationship, because women can smell that easily
2. Never respond to that message, because responding will only make you sink in the quicksand more.
3. Be careful about what you say, because that can trouble you in the future.
So now what do you do from here on out? Forget about her, move on, and don't even talk to her anymore. Start flirting with and going for other women, and set your focuses on other things. If you see her in work, give her a wave and nothing else. If she tries to talk to you, cut the conversations short but keep them interesting. If she does come back around for you latter, then the ball will be in your court, and you will have a better shot at her.