You might consider a phrase from the poet Quintus Horatius Flaccus, known in English as Horace, suitable for your purpose. I don't have any tattoos, but if I were getting a Latin one, I'd want a real Latin quote from a classical author. There is something almost incredible about hearing the words from the distant past speak so clearly.
From Odes 3.29
Memento componere aequus.
As with most Latin, it can be hard to put into English succinctly. Roughly it means:
Remember to settle things calmly.
Aequus is an masculine singular adjective meaning calm, even, unruffled and modifies the imperative verb memento. Componere is an infinitive that follows memento and means to gather, settle or arrange.
Another possible phrase from Horace, Odes 2.3 is:
Aequam memento rebus in arduis servare mentem.
Remember to keep a calm mind in difficult
This could easily be shortened to:
Aequam memento servare mentem.
Remember to keep a calm mind.
These phrases are each from one of Horace's lovely odes. He lived during very turbulent times, as the Roman republic transitioned to the Roman empire. His philosophy was to live in the moment and enjoy what you have. Tomorrow is an unknown. When Horace uses the adjective, aequus, to describe a person or his mind, he is not speaking of controlling anger, but of maintaining calmness in the face of human uncertainty in a more general sense. Perhaps, this more general sense of calmness will speak to you. Perhaps not. Either would make a classic tattoo, in my opinion.
That said, if you like Horace and want a phrase that speaks specifically about anger, you might consider this one from his letters.
Horace, Epistulae, 1.2.62
Ira brevis furor est.
Anger is a brief madness.
If you would like to read more about how anger was viewed in the ancient world, check out: Restraining Rage: The Ideology of Anger Control in Classical Antiquity.