TBH you can, it's just not easy. It is totally possible to take control over your emotions and reactions. The more high stress the moment is of course the harder it will be, but you practice with small changes first. You may not change how much something sucks, but you can change the outcome of a great many situations if you practice even a little mindfulness and control over your reactions. Sometimes simply being aware that you are acting erratically is enough to remind someone to regain control over themselves.

Controlling our reactions and emotions is simply part of life. There are healthy ways to respond to things and then there are unhealthy ways. IF we didn't have some form of control we would all be 2 year old toddlers throwing temper tantrums because we don't want to do something. If you got past the phase of stomping your feet and screaming because someone got 2 cookies and you only got one you have already demonstrated you learned to control your emotions and reactions to some degree.

Notice I'm not saying you need to stop being angry, or upset, or sad, or hurt or anything else. However, learning to acknowledge what you are feeling and respond in ways that reduce your own stress is more helpful than responding in ways that create a feedback loop of emotions (you're crying because you're sad and sad because you're crying...) or increasing the stress of the situation (car broke down in the middle of the highway? That sucks! but getting to the side of the road, taking a few deep breaths, calling the tow company and making sure you are safe is far more beneficial that slamming down on the horn in frustration while screaming in your car and stomping your feet). SO yes you can calm down at will, you just need to practice the skills to do it when you need to.