Imagine you are at home, everything is fine, you're watching TV or reading a book. How do you breath? Take a few seconds to see yourself in that situation and breath just as you would breath there.
Now imagine you are late for rehearsal and stuck in traffic. How do you breath there? Not quite the same...
In concert or rehearsal, try to breath like you would in the first situation whenever you're not playing. By doing this, you are sending the message to your subconscious that everything is ok which will calm your mind a bit and help you focus on the moment.
You can do this in any situation whenever you feel nervous or irritated. Just breath normally and calmly and your mind will follow.