Daily meditation

I'm no expert on anger management by any means, but this year I started doing 10 minutes of daily meditation every morning, and I can say that I've definitely felt the difference it makes in generally managing negative emotions and unhelpful reactions. Regular practice at quietly focusing on breathing, while also then noticing when thoughts and feelings start to bubble up, neutrally observing that they're there, and letting them go, has helped me to also sort of automatically notice negative emotions the same way when they come up during the day (frustration, impatience, perfectionist reactions, etc.) and treat them as things that I can observe happening internally and that aren't "me" (if that makes sense)—which does a lot to rob them of their power. Plus I just generally feel calmer, more balanced, and better able to focus throughout the day.

(This more or less describes what I've personally been doing, although that's not the only way to do it. The book I linked here is a generally interesting read that addresses some of the philosophy and scientific basis of it.)