I have dealt with this before, too, as have many others. The good news is that it is totally fixable and you both will be happier.
The rider can get to a point where the leg stays on without her even realizing it, maybe. One thing I found that helped was to adjust my leg cue from “push” or “squeeze”, to a “tap-tap” motion. You must have a relaxed leg to do this, so I found it helped my equitation, plus it is a very clear and definite cue to the horse that is different than the feel of your leg just resting on their side.
You might do this for a few weeks, while you both readjust the idea of the “GO” button.
If you tend to hang on with your leg, or the horse tends to lean on your leg (two sides of the same coin, maybe), this might be helpful.
You said about how you were working much harder than your horse. It takes a lot more mental energy to sit still and make the horse do the heavy lifting :lol: