13 years are more than sufficient to let go. Something must be terribly wrong here. Could you explain more about why you are having a difficult to forget about this person and move on to much better things in life.
I agree with Ask Oracle after 13 years surely you have realized that it is over and it is definitely time for you to start another life. Besides that being the signs that you are, you were not compatible in the first place so it is without doubt that your marriage did not last.