I would recommend "Mars and Venus Starting Over" by John Gray. Good book on helping you realize that the old R is dead and you need to move on. Only by letting go and living your life can you hope to attract her back. And once you get your life in order you may come to realize that she is no longer right for you and therefore you cannot be right for her.
It is hard to let go but acting "married" only tells her that you are conquered. That is not attracive in a man. Attractive men are confident and in control (not controlling) and the attractive man does not need a relationship - it is only a desire.
Also notice the OW you speak of "who is sweet and wants to love me, but I am having a hard time letting her". Ask yourself why she is attracted to you. Is it b/c you are pursuing her and waiting, hoping for a R (as you are doing with xW) or is b/c you are making yourself less available to her?
