Breed success with success. High-five all aspects of your child's experience --getting fit, learning new skills, being a good sport -- to inspire her to keep trying. You can say, "You were a great teammate today. I'm really proud of you."
Analyze, don't criticize. Instead of scolding, ask your child why she didn't overrun the bag or what she might have done better.
Lower your expectations. We're so used to watching pros, we may expect our kids to have the same focus and discipline. You can dress them up in cleats or shoulder pads, but they're still goofy 8-year-olds.
Let her quit. If your child insists, taking a season off at 7, 8, or 9 isn't the end of the world. Kids grow so fast that there's little point in specializing in a sport that young anyway.