Try a Cognitive Behavioral Therapist. Psychiatirsts will only medicate.
There's a book called Authentic Happiness by Seligman. Read it cover to cover and do all the online excercizes (you can register at his website). There's even a chapter on forgiveness, which is key to not obsessing about OW. Great stuff.
Here's something that might help.
The key is to get some hobbies in your life that consume your attention so completely that if even for an hour a day,you can NOT think about her, it will begin the process of healing.
Painting? Karate? Rock Climbing? Ball room dancing? Fencing?
Notice how they require extreme focus? They consume your attention. They free your mind to focus on the moment. Running or Biking are good for the health, but allow your mind to wander too much. When I was finished with a bike ride, I often was furious and upset.
