Free yourself from the self-defeating energy of negative thoughts about someone.
Think loving thoughts about someone you resent, wishing them good fortune and blessings. Pray that they receive all the wonderful things you want for yourself in life. Say it out loud, “Hashem: please give love, health and peace to [whomever] today.”
Do this as soon as possible after the event that brought the negative feelings. Or at least before you go to sleep. Don’t let those negative thoughts fester. Release the resentment.
If you do this consistently, you will realize that where you used to feel bitterness and resentment and hatred to others, you now feel compassionate understanding and love.
(Even when you don’t really want it for them, and your prayers are only words and you don’t mean it, go ahead and do it anyway. Try it for a few weeks. You’ll come to really mean it and to want it for them.)