Hey {Recipient},
Thank you for your efforts at today's {game/tournament}. Our team may have lost, but at the end of the day, what matters is that you gave your 100 percent.
I'm proud of how far you've come since the season started. You've never missed practice and are always eager to learn new {moves/techniques}. Keep that positive attitude up, and you'll succeed not just in {sport} but also in life.
{Your Name}