It may conflict with everything in your brain and heart... but the EASIEST thing to do I have noticed, is to:
Completely cut them out of your life and HATE them.
Your hatred will soon fade into nothing anyway, so you may as well feel better earlier by just hating the bitch.

You may want to hold on to sentimental memories and possessions etc, you may think you still want to have sex with her... you may feel sick at the thought of her having sex with other people.
So to counter this, HATE her, and forget she exists.

Seemed to work for me...