Anger is how you feel when someone doesn't hear you or understand you. The closer you are to that person, the more it'll be difficult when they "persist" in not understanding you. Sometimes it can be a question of you, not communicating properly though. Or both having too different points of view to understand each other.
There's one person with whom it's been difficult for me to stay angry, or have clear feelings, or "let go", although I would have several reasons to be angry at him. I usually don't put up with bs from people, but with this one person, communication has been... let's say "not up to it", for clear explanations. So I stayed with unresolved feelings, but it's not anger. It's a mix of things. Questions with no answers, to which I can't get satisfied with my own hypothesis, I want the truth. But sometimes people don't care to give that to you.