If she has an older sibling, get head gear and boxing gloves and go to town! Just kidding, but based on my experience kids that have older siblings that they play with has no fear.

My middle son use to play pick up with his brother and his friends (4 years older) and when he played in his own games, he gave zero shits about the size or skills of the opposing team.

In short, find places where she can play pick up with older, faster players, even mom/dad can go out there to challenge a few balls. Best of luck.

Training repetition will remove the bad habit.