Don’t let your competitive side get the best of you. It’s fun to win. It sucks to lose. But keep things in perspective. Unless your livelihood or your family’s livelihood depends on you winning this game/earning the contract (whether in sports or in business), it’s probably not worth losing your cool over.
Don’t flip the monopoly board, throw the controller, or punch a wall because you lost. Everyone loses something in life. That’s just how life works. If you’re not losing in something then you aren’t pushing yourself to play against the proper competition.
When you lose, lose with dignity. You’ll maintain respect. If you lose and act like a child? People will lose respect for you.