Intrusive Thought Elimination
When you have an intrusive, unwanted thought, understand that you can't control what pops into your head, but you can control what your mind does with it once it's there. This is a big key to success. Understanding that, you choose a positive thought - one that brings up a good feeling in you. Picture it in your mind and sit with it for a minute to enhance the positive feeling, and give this picture/feeling an affirmation that goes with it. IE: picture a warm walk on a sunny day and say to yourself, "I'm on the path to recovery and I know I'm going to make it." Every single time one of the unwanted thoughts pops up in your mind, exchange it for your chosen thought. Do this every single time no matter if you have to do it 100 times an hour. Do it without fail. What you'll accomplish is to re-train your brain and you'll see that your intrusive thoughts will trigger your chosen thought, which will give you relief.
Talk to your friends, read your books, exercise and be good to yourself. You should be proud that you haven't gone back to her. I suggest that you stop reading her emails. :)
NC does wonders for those who want to detox!
You're going to get through this - you just need time and distance. Hang in there and put one foot in front of the other. Find yourself.