Why does wishing goodwill to a difficult person ignite bitterness and mood swings in Metta?

The Metta practice challenges me to wish someone a good day. However, when I imagine them, I do not feel calm--I feel bitter and furious. This aggression leaves me in a bad mood the rest of the day. I fear that my attempt to insist on the good wish is simply making the bad one more noticeable and worse still the session has caused me to show extreme mood swings. Is this outbreak of hostility an indicator of the release of ancient trauma? My name is Varun, DOB 22nd November 1991. Time: 7:30 PM. Location: Bangalore.