Holding a grudge is keeping a receipt for something you can never return. It just clutters the wallet.

Resentment requires maintenance. Keeping the account open means rehearsing the details, checking that you still have them right, and giving the same event fresh attention years after it finished, which is a considerable amount of labor for no possible refund. The other person is rarely aware any of this is happening.

Notice how much room a specific old grievance is currently occupying. You do not have to declare it resolved or call anyone. Simply stop rehearsing it, one occasion at a time, and see how much space comes back.