Saladin forgave the Knights Templar for their vandalism and murder, and let them go on their way as reformed and happy and sane individuals, despite their past criminal track record. Given that, you can forgive someone who has offended you or insulted someone you care about.
Lao Tsu noted in Tao Te Ching that “Those kind, be kind to them; Those unkind, be kind to them also.” and there is also the aphorism “keep your friends close and enemies closer.” from the Godfather part II. Finally, there is the old adage that “To err is human. To forgive — divine”.
Your time is likely too precious to hold grudges against your friends, your past friends, or even people who currently dislike you.