Every person has moments of suffering and unpleasantness in life. If you master the skill of living in the present, you will keep these moments limited to the actual negative experiences. Both before and after a painful experience, you will focus on what actually is at that moment, freeing you from much unnecessary pain in your life.
Very young children have this skill naturally (we all have it when we were younger), and that is why they enjoy life, unless they are presently in pain.
As we grow older, our ability to use our minds and think about the past and future increases. This ability can be utilized in very beneficial ways, but it can also be used in a detrimental way. We can transform our lives into suffering and torture by keeping in mind all our unpleasant experiences of the past.
Forgetting those experiences is the positive aspect of forgetfulness.
(Chovos Halvovos; Rabbi Pliskin's Gateway to Happiness, p.145)