Why does extending loving-kindness to a challenging person leave me drained and angry?

When I manage to give loving-kindness to someone who is challenging, I am as drained as soon as I do it, like I emptied out my energy levels. I am so tired and angry because of this emotional burnout. I fear that the exercise is emotionally straining my system, and that I have to pay a spiritual price of letting go of resentment. This burnout develops an increase in my general anxiety and mood swings. My name is Karan, from Mumbai. DOB: 12th January 1988. Time: 6:05 PM.