Metta is the practice of repeating a small blessing, first for yourself, then someone you love, then someone neutral, then someone difficult, then all beings.
The phrases are simple: May I be safe. May I be well. May I live with ease. Repeat them slowly. Let them do their small, patient work.
Why the difficult person matters
Sending goodwill to someone you resent will feel dishonest at first. That's the practice. You are not endorsing what they did — you are refusing to let carrying them cost you your peace.