All people definitely can do this. They just choose not to.Wrong. The classic demonstration is Phineas Gage. A man who by all accounts had very good self-control until the day that an accident drove a railroad spike through his brain, taking out the frontal lobe that is responsible for said control. After that he lost control. Not through lack of will, but because of physical trauma. The same phenomena has been observed in lots of stroke victims.
Moving on, there are a ton of psychiatric conditions which are not even that rare that result in people who have hardwired reactions to words that are out of their conscious control. PTSD. Social anxiety. Manic-depression (at either extreme). And so on. These days we can map out how words get processed and see the short circuit that keeps the frontal lobe out of the loop. It isn't a question of willpower. We know how to treat these conditions and help people regain control. We don't do it by telling people to "try harder".
What about normal people? Well it has long been known that any normal person in the grip of strong emotion also loses control. It doesn't matter why they have the strong emotion - someone died, you were in a fight, etc - once you feel the emotion, you have no control. Furthermore every time we go through this pattern, we wind up laying a path that makes it easier to go through the pattern.
Afterwards it is easy for people to go through a thought experiment. Say, "I should have done this, said that, held my temper." Blame themselves. Promise they will do better But guess what? They are wrong. Next time they don't do better. Not because of lack of willpower (though that may be an issue), but because human brains don't work in the way we consciously think they work.
But note this is not just defeatism. Because it turns out that we can understand these classic patterns we have. And with understanding comes the ability to get better control. But not through having an iron will. Instead through having the self-awareness to know, "When I feel this and hold it in like that the next thing I know I lose control. So I need to pay attention to this early sign and walk away for 10 minutes, then come back and continue."