Almost everyone who wants to hold onto resentment will have plenty of reasons and justifications to explain why they are resentful.

But a mind full of gratitude, kindness, and joy will not want resentment to rob them of their happiness. Conversely, a mind full of resentment will be full of happiness-robbing thoughts and emotions. In short, happy people have no room for excessive resentment, even if justified. And resentful people will have no room for happiness-producing thoughts.

So it's a matter of choice. Happiness or resentment. It won't be both.