A problem with resentment is that even if rationalised it can sit and fester. Sometimes you don’t even know it’s there.
Look for clues.
Imagine my brother did something that really bothered me – like killed my cat – I might know it was an accident and be able to forgive him, but the resentment is stored in me; somewhere deep. In the animal that feels things
I could scream and shout and pummel the brother problem. Why did you do it?! You fucking idiot!? You selfish bastard?! I could dump it all on him.
That might make me feel better; but likely all you do it translate your resentment to your brother. He then holds that shit and likely resents you for the things said that can’t be taken back.
I have heard of “growling” therapy, which appeals to me and in which you get the subject of your resentment (or other negative emotion) and all you do is animal growling and clawing at each other. I guess kids do this. The idea, I suppose, is to release the energy held by the inner animal – maybe the limbic brain.
Another option is that you work the scenario out on a ring-in. That person brought in plays the role of the subject of your energy whirlpool and you let it out at them. I have witnessed this; it was hell scary. Importantly, after releasing your aggression at the willing actor, you have to switch and become them. Attempt to embody them so as to fill the animal with empathy.
We do this instinctively don’t we? Every time we act out on someone that isn’t the person we are really angry with. We might hold resentment for a lover that jilted us, and then play that out on our next date.
Again, look for clues. Get someone else to spot your clues by giving you feedback. And never let it fester if you’re able.