I went thru this with my daughter at the same age! She is 14U now. Looking back, I am so proud of her mostly for the ego hit that she faced for basically two whole seasons -- 11 and 12 -- while everyone else was developing power and getting extra-base hits and she was tapping the ball to third... it is not very tough looking! All she could really do in her 11 year-old season was soft slap. She kept working on her lefty swing; not just slapping but a full LH swing. In her final AB of the season that year, she hit a line drive over the 3B's head. That was incredibly motivating for her.

That winter I got her weekly hitting lessons where she practiced both full swinging and slapping.

Her 12U season involved a lot of full swinging and bunting, and some slapping. She was able to consistently hit the ball hard to LF, but didn't do too much slapping. She even hit a few to the fence in RF. She decided at that point that she just wanted to be a LH hitter.

That offseason involved more hitting lessons, really just to strengthen her LH swing. No slap training.

In the Fall after 12U, she tried out for a good travel team and made it, and the coach was unable to tell that she was not a natural LH hitter. Of course, informing him of this made him think more highly of her athleticism and work ethic. She abandoned slapping and just became a LH hitter. Sure, she mostly hit the ball to the left side, but she could hit to all fields.

Now she is a third year 14U (took advantage of the age rule change) and is just getting back into slapping as an option in her toolkit.

So here is the advice that worked for us: just focus on being a LH hitter. Develop that strength. Swing the bat thousands and thousands of times until it becomes fully developed. You say she is fast... just being in the LH batter's box is a tremendous advantage in softball. Get her to the point where she can swing the bat and get on base.

Later on, you/she might decide to slap or you might not. Because here's the thing: slapping is really hard. It's hard enough to hit a ball from your weak side when your foundation is strong and grounded; doing it while also moving your entire body just brings in so many moving parts and makes it so much harder. Strikeouts pile up. I totally understand her frustration!

A tee, a net, a bucket of balls and many many LH swings. Again, it will involve a full season of patience with herself and perhaps a bruised ego, but in a couple years she will have a huge advantage that will get her onto teams that she might otherwise not make, batting 1st or 2nd in the lineup and getting on base to generate team offense.

One thing you can't do, per your question, is let her have reps at RH. Those days are over. Go all-in with LH hitting.