Things come at you from the outside. From the inside, too. All day long.
You don’t control your reactions to them. No matter how vigilant you stay, there are buttons inside you just waiting to be pushed. You can’t stand in the way of them like you would a speeding bullet.
But you do control your responses. Or rather, you can, if you choose to.
You can’t help getting mad at someone who lets you down, for example. But you can help staying mad at them. You can help holding a grudge. You can help using that person’s little digression as an excuse for you to be an asshole.
Own your responses. Practice them. Perfect them. And let go of your reactions, which you never had any chance of controlling in the first place.