Brooks Koepka played with a lot of poise to win the U.S. Open at Erin Hills in 2017 and Shinnecock Hills in 2018. When you think about it, it is an amazing accomplishment to win a U.S. Open, but to win them back to back is incredible. The U.S. Open is the ultimate test of golf. It is usually the most challenging major to win.
Brooks is obviously an unbelievable talent. He is strong and athletic but that is only a minor part of why he can do what he does. Brooks has an innate ability to be calm. He doesn't let anything get to him. It is his key to success.
If you watch enough golf, you will observe there are a good amount of tour players that keep their emotions in check. They have great self-control. The two that stand out to me that are on next level of great self-control are Brooks and DJ.
Its the winning mindset that they naturally have. Just watch these guys. It sometimes looks like they aren't even playing for anything. There doesn't seem to be any stress, pressure, frustrations, anxiety, etc when you watch them hit a bad shot.
I would recommend observing and watching Brooks as much as you can. Try to pick up on little characteristics of self-control he exhibits and try to emulate them yourself. Put it into practice. Keep self-control in mind and over time it will get better and better. Remember, the game is 10 percent physical and 90 percent mental. Put in the mental practice to get better scores. Brooks has the mental game figured out, watch him and learn.