Actually, you can relax a bit, as neither is possible:
- Since it is not you that controls your anger or other emotions, the idea of mastery is at best a distraction. If you controlled your emotions you would allow only positive emotions to occur, but in practice they are very mixed and often unpredictable. So it seems that you are not the owner.
- Since you and your anger are not in reality two separate things, your anger can never overwhelm you. What you call your anger is only what you call a particular mental and physical state.
Nevertheless, anger is an unpleasant sensation which clouds your judgement, may cause guilt and often has negative social consequences. So the question arises: what can you do about that?
In practice, when you fully understand the nature of reality, anger does not arise. This may be surprising, so I will try to clarify.
Anger and other forms of stress, are the result of wanting reality to be different from what it is. But of course it never is.
What happens happens. Nothing else ever happens.
This is simple, but also radical. For me it is a relief to discover that, simple though it sounds, understanding how this world really is and how it really operates turns out to be enough.
No effort is involved. In reality, there is nothing to be mastered and nothing that could possibly overwhelm you here.
Best wishes for a happy and peaceful life,