I too have a very difficult time holding grudges. Usually it starts first thing in the morning with angry racing thoughts in my head. Or constant thoughts throughout the day. What scares me is knowing there are people out there who hold grudges against me too.
As for forgiveness, I can be very forgiving. There was this one guy who insulted my mother. I held it against him for 30 years. I would seeth over it. One time I ran into him and straight out told him I was holding a grudge against him. He was extremely sorry… very sorry… he had no memory of even insulting my mother. I can honestly say I totally forgave him and to this day have absolutely no animosity towards him over the incident. So I know forgiveness is possible.
What it might be with grudges is unresolved conflict over people who have hurt us.