Anger is usually a signal that something needs to change. And it is most frustrating when we can't facilitate whatever that change needs to be.
I think you have to acknowledge it before you can move through it. I think, like all emotions, being present within your anger, actively acknowledging *what* it is about a situation that makes you angry... and sometimes, grieving those circumstances, is all you can do.
I don't know if it will help, Ephie, but there's a good book titled "Ambiguous Losses" which really addresses the situation of *not* getting a healthy sense of closure when our relationships with those we were close to change for the worse. (Some of this focuses on physical illness, such as a parent with Alzheimers; where the person we knew before simply "isn't there" anymore.) The reality is that sometimes, we don't get closure or get to say a real 'goodbye', but that person we knew doesn't exist any more, either due to aging, illness, addiction, mental health issues or even simply 'not knowing' (as in MIA soldiers or families that emigrate who never hear from loved ones again).
Being aware of anger is important. And at some point, we have to take a step back and figure out what *our* options are within that frustrating situation and decide (when we aren't angry) how best to move forward. I am sure you already know this. This is what helps me when I am furious-- to be aware of the feelings, to allow myself to feel them, and then--after that good, hard, ugly cry (which is good for us), to accept the situation and then really look at what my choices are. Even *not making a choice, just gonna sit with this* is sometimes a good choice. Sometimes, honestly, my choice is just "I can't do a damn thing about this so I'm going to go take a walk and re-center myself". (As others have suggested, physical activity can help.)
Lastly, when we feel most helpless about a situation, our anger is probably worst of all. In those cases, I try to find other little things I *can* do to make our lives better, even if they seem pretty insignificant. A couple of years ago, a brother-in-law of mine was killed in a traffic accident. Over half of our family didn't show up or even send condolences to my sister, who had become a widow with five children. It was in doing *what I could do*, no matter how insignificant, that helped me to feel better. I couldn't control the actions of others, but I could help in little ways and that's what I tried to focus on.
Sending you strength and little moments of peace.