Pin down what makes someone a sore loser. Getting disproportionately emotional, feeling entitled to a victory because you tried, feeling the need to make excuses or shift blame, "over the table" grudges, etc. Nobody likes a sore loser, but they only show their colors when defeated or when they believe they are defeated.
And now do your best to notice and avoid these same symptoms when it comes to forming relationships. Take any outcome gracefully and do your best to keep your emotions, words and actions consistent with each other.