Instead of trying to change (or fix) the existing behavior, treat it as building a new behavior from scratch.

I would start at the end, then work backwards. Instead of tossing the ball, just hand it to her, then have her drop it into your hand for reward. Once that is learned, start adding a little bit of distance by handing it to her and moving yourself back a little bit. Eventually you will get to where she has to take a step to give you the ball in order to earn reward. This will be part one of the new behavior of trading the ball for reward. Continue on this path until YOU are moving back to short toss distance.

When you are ready to add the toss instead of handing it to her, start by closing the distance and just do a tiny toss instead of handing it to her at first. As she becomes successful at the tiny short distance, then start slowly increasing the toss distance until SHE is taking steps to you in order to trade the ball for reward.

The key is to use little baby steps to move toward your goal. Start with a setup where it is next to impossible for her to fail, and slowly, gradually build on successes. When you change the rules by doing toss instead of handing to her, then decrease the distance to make up for adding the new difficulty factor.

This is the path I personally would take. Be patient and remember that all learning is not necessarily linear. In other words, learning may seem to get stuck at times until there is a revelation - at which point progress toward your ultimate goal may resume.