Yeah, what Nonsensical said. I like it!
I think the prayers for that person have to be ongoing until the resentment goes away. The resentment doesn't miraculously go away with one prayer, in my experience. It has to become habit, just like my previous resentments were bad habits.
The only person suffering when I'm mad at someone is me. It's all in my head. They are going about their lives with not a care in the world as to how I'm obsessing.
If I wish them well, I've truly done all I can do about it, it is the right thing to do - and I have let it go.