This has worked for me.
Use an adult tennis racket and a bucket of tennis balls. Swing it like a bat. Do soft toss and let her wail away. Full extension and good technique but mainly enjoy the ball sailing to the fence. I suspect that she trying to remember all the stuff you are telling her and attempting to be perfect. (a common ailment of young women) the tennis racquets are extremely light and have a very forgiving sweet spot. Buy a bucket of tennis balls and just hit the bejesus out of them. Try very hard not to criticize her technique let her drive those balls to the fence and beyond, let a little joy creep into her life. As she improves you can switch to a very light bat (for speed) and still use the tennis balls, every one can feel like Babe Ruth. (We use tennis balls indoors for my high school team and they really look forward to scrimmage, no gloves and everything is in play, chaos reigns and happiness abounds)
Remember that softball is mental game, you can trick her into swinging hard, I always find that joy is a great motivator.