Dressage lesson last week brought to light that my lower leg sits too far forward, and I need to do some exercises to help strengthen my lower leg to help keep it in place.
Check that you are not sat too far back in the saddle either. If you could post some pics/vid it may help us to give suggestions.
What did your instructor suggest?
And that the saddle sits where it should and is balanced ie not tipping down at the back as that might throw your legs forwards. Have a look at it from the ground to see if you're going to be able to sit tall and straight in the deepest part of it