Why am I still angry years later? How can I stop being angry?

So a few years ago this artist that I considered myself to be civil acquaintances with (never really were friends, but were more than strangers), had a bit of an issue. She is a fairly popular artist and inspires a lot of people. One young artist really looked up to her and tried to emulate her style, by using the tutorials she had made for others to use, teaching them how to draw in her style. The artist was really mad about this person emulating her style, and made call out posts about it. She had over 6k followers, and this other artist had less than 100 followers. She posted about them on twitter and Instagram where she had 10k+ followers, and they all started to harass the littler artist until they took their art down. She then proceeded to talk crap about me and our mutual friends for not taking her side in the matter, going so far as to say that we supported theft.

Since then we haven't talked or had any interactions, but any time I see her my blood boils and get extremely angry all over again... it takes all of my strength to not call her out and expose her for being the jerk that she was... Why is this and how can I get over it? I'm really tired of reacting this way anytime I see or hear about her.