For those who have mastered serenity, fifteen seconds ago is ancient history. They realise that once something is over, it is over regardless of whether it has been over for many years or for a relatively short time. It is understandable that it can take different people varying amounts of time until they are able to let things go.
The goal should be to let go of what is finished. In truth, it is gone whether or not you let it. It is just a question of the degree of emotional mastery that you will have. Regardless of where you are at this moment, you can always improve on your ability to let things go as soon as they are gone.