Shake their hand when you lose and mean it. Bitterness is a second loss you choose.

Losing takes something from you once. Carrying it as a grudge takes something again, daily, and unlike the first part this one is optional. That is what makes bitterness expensive. You keep paying for a game that finished hours ago, while the other team has already gone home and forgotten it.

Do the handshake before you have processed anything, because waiting until you feel gracious means it never happens. Say one true thing about how they played. Then go be angry somewhere useful, like practice.