I was the same. I can't easily describe what changed, but now I don't play to win or lose, I just decided to play for the sake of playing.

If you lose a game, just stop playing for a bit, and don't give in to the feeling of wanting to play another to prove you can win. This will only result in frustration.

If you lose, make yourself think "That was a fun game. I learned X, and will try and do Y next time."