"When we hate our enemies, we are giving them power over us: power over our sleep, our appetites, our blood pressure, our health, and our happiness.... Our hate is not hurting them at all, but our hate is turning our days and nights into a hellish turmoil." - Dale Carnegie
It can be very hard to let go of a grudge, but if you can, it will make your life better, which makes the lives of those around you better.
An extreme example is the classic Romeo and Juliet story. Two families are feuding and ruin the lives of the star-crossed lovers. Family feuds have been happening for a long time, but to what point? Just anger, hurt and pain.
John-Roger and Peter McWilliams have some words to help: "Whenever he thought about it, he felt terrible. And so, at last, he came to a fateful decision. He decided not to think about it."
Don't spend all your time dwelling on your troubles or trouble is where you will dwell. You do need to acknowledge and work through your negative feelings, but don't let them linger, let them go.
The best way to deal with an enemy is to not deal with them at all, including in your head.