Ask yourself what winning the game gains for you and what losing costs you. Then ask yourself what behaving like an ass when you win or lose might gain you or cost you. No one will remember you won that particular game in a month except maybe you. It is not going to be something that other people findly remember about you when you aren't around. However if you act like a ass, they definitely will remember and what you will gain if you repeatedly behave like that is a reputation, and not a good one. Nobody else really cares if you win or lose. It says literally nothing useful about you in day to day life.

What do you prefer to win, a board game or a lasting relationship.

Now that's said, when you are in the midst of losing and you can feel the tension. Remind yourself what is really at stake as above.

If you start losing and winning graciously you will hopefully come to realise the above naturally.