Anger is what I feel when something happens that should not (in my view) happen.
Resentment is when I continue to feel anger past the point of impact.
Anger is like a fire-alarm going off.
Resentment is leaving it on past the time it's informed me that there is a fire.
Resentment is dumb. It's also delusional: I'm feeling in response to something that is not happening. I'm not responding to the event but to what I am currently thinking about it. This is called fantasy.
So why resent?
Well, it gives me the feeling that you're being called to account. Which you're not.
It gives me the feeling that you're being punished. Which you're not.
It gives me the feeling that I'm being defended or protected. Which I'm not.
It gives me the feeling that I'm forefending future ills. Which I'm not.
It has a number of other disadvantages, too.
A quick solution:
I'm angry not because of you but because my demands are being threatened.
Drop the demands: having them doesn't help, and met demands do not produce happiness anyway.
Drop the condemnation: no one appointed me a moral arbiter.
Foster empathy: jerky behaviour comes from being dumb, ignorant, illogical, irrational, selfish, careless, or mean. I've been those things too. We're the same.
Foster love. Imaging the Higher Power's love coming down to you, through, you, and out to the world.