You are missing the cues she is giving and reacting too late.
Horse may quicken its step, and more than likely is putting one ear forwards and one back to see if you are paying attention to it.
Be an active rider. If you feel her lighten up, then move heel against her, or tell her to knock it off. Take up on the reins. Stop her and make her back up a couple of steps. But quit sitting there and waiting until horse has its head down and then trying to fix it.
Once a horse has its head down, you have lost the leverage point. It is hopeless.
You have to do something before that head drops.
Watch the horse's ears, and when you see one forward and one back, either touch with heel, say QUIT, or stop horse and back...but you can stop this by stopping it before it happens.
Horses make me a better person.