Being angry is not a sin. Carrying it around, holding onto it is the sin.
I get angry. I get upset. And I have boiled my insides by being angry without effectively dealing with it.
Many times carrying around hot anger burns only me, and the offender isn't even aware. What good is that?
I am by no means an expert on effectively dealing with anger but one thing that I do know, if you tell the other person that you are upset and ask if they are willing to discuss the problem, you will instantly feel somewhat better. Even if they are not willing, you know that you have thrown the ball into their court and now it is on them and not you.
But if you can sit down with someone, (the car is a great place or a restaurant) calmly state your true feelings and why you feel hurt, and then LISTEN TO THE OTHER PERSON as they speak their heart, it will go a long way toward having peace.
Sometimes people will ignore you or not want to hear you. I have found that the best response to that is: Well, I feel hurt and would like to discuss it with you when you are willing.
Holding onto anger builds resentments and grudges.