I agree with the "try sitting" - the rider needs to be certain they are not leaning forward (even a little bit) - rather sitting straight up, as you ask for stop press down (NOT forward) with BOTH stirrups, brace you back and do NOT relax at all until you have reached a complete stop.
I ride dressage so I disagree with removing your legs from horses sides since to get a good stop hind legs MUST come under horse (keeps horse from falling on nose - and this is of course falling forward).
So keep legs on horse (resting there not squeezing), tilt hips forward driving horse into bridle, press down hard on BOTH stirrups, lock elbows at waist, stop "following" horses movement with hips/body, squeeze both reins, and exhale (makes you seem "heavier" to horse). Try this at walk, then trot/halt transition (don't go further until these are perfect), then canter/halt transitions.
I'd also think about horses teeth - they need to be done at least yearly, sometimes more often. If her teeth are bothering her she could be locking her jaw to prevent the bit from "hurting" - making the stop harder to achieve.