Train, for me forgiveness is a decision. It is bring joy and love into your heart and moving forward. As Starsky said, we all make mistakes. While we may look at someone who has an A as making the biggest mistake of his or her life, in reality, they may see things that we did to be just as bad. I am not saying that they are and that we are not to blame when someone has an A, but none of us our perfect.
I also think life is too short and we all need to forgive and move on. Holding on to grudges is dangerous for your help and soul.
I think in the case of the Ds that you witnessed, I am not sure whether or not a lack of forgiveness was the root cause. I see it as one or both spouses just not deciding to work at marriage. I believe that any M can be awesome through W. But not if one spouse does not want to be married.
I feel good/better when I bring God's love into my heart and view the world with that lens. I pray that my W makes the decision to work on our M. That is all that I can do. But I already have forgiven her. I just cannot go through life holding onto a grudge.
Me: 40, W: 40 M: 15, T: 18 D - 10, S - 7 D announcement 6/7/2014 A discovered 7/20/2014 (but denied by W) Still living together and sharing same bed