Here are some tools that can help your youth players focus on what they can control.
Effort goals (such as sprinting after loose balls near the touchlines in soccer or finishing receiving routes in football) are more under your players' control (if they are working hard) than outcome goals. Set effort goals in addition to outcome goals (winning at least half the contested loose balls in each soccer match or making at least one reception in each football game). In this way, over time, if players achieve the effort goals, they will move toward achieving desired outcome goals as well.
Targeted Symbolic Rewards
After each game, recognize one or more players who worked very hard or who completed an "unsung activity" that you want to see more of, such as tightly marking an opponent on defense in soccer, or, in football, running receiving routes hard all the way, even as the quarterback’s second or third option. Make sure the reward is symbolic (not money or something that is of value in and of itself). Some coaches give out "game balls," while others give the "dirty shirt award" to the player who hustled consistently throughout the game.
Team Mistake Ritual
Fear of making mistakes can negatively impact performance. The way in which coaches deal with players' mistakes may be the single most important thing they can do. Adopt a physical team ritual for players to use to get past a mistake, so they can focus on the next play. A good time to use this in soccer is just after a missed shot on goal, or after a bad pass that went out of bounds. If football, players can use this after a missed block or tackle, or a dropped pass.
Talk to your players about what ritual they want to use. Some players like "no sweat," signified by running their fingers across their brow, while others like "brush it off," signified by pretending to dust off their uniform. Some teams make a fist and then open it to "let the mistake go." Rituals remind players that mistakes are inevitable, and the most important thing is how players react right after their mistakes.