I had that moment of holding a grudge against some people; a moment in which I learned how to let go of my own anger because it simply was not worth keeping for. The story of how I struggled about this matter was provided here. What I can say about not being able to be mad is this; staying mad at someone is tiring and along with time, your anger will simple calm down to the point where you can realize how stupid you are when holding a grudge. In the end, if you let yourself cooling down, you will eventually be able to let go your anger and grudge.
I did and now I am still learning how to make peace with both myself and other people.