I have this 19 yr old mare and she is "all go and no whoa." What can be done to to calm her down?

Because of her age and experience it sounds like a behavior associated with fear from pain. I suggest you have her teeth looked at by an equine dentist to find out if this is the case. You will be amazed at all the soars in the horses mouth when there teeth are not wearing evenly.

Now if pain is not her problem then to teach her your cue for stopping place a branch, stick, pole, board or a piece of rope down that is about 8 feet long perpendicular to a fence inside a round pen or arena. Ride at a walk along the fence and ask your horse to stop about 10 feet before you come to the stick. If she stops before the stick then great. Sit there for 30 seconds and turn her around and go the opposite direction. Stop again before you come to the stick. Again if she stops great and sit there for 30 seconds. Do this several times and then progress to the trot doing the same thing. If she doesn't stop before her feet cross the line then when she does stop have her back up until her front feet are back over on the other side of the stick and then wait the 30 seconds. What this does is teach your mare to learn that she will be stopping at the same point each time. When she does learn that she will then be associating what your body movements are right before you stop and so she will pick up on those cues when you start stopping her at other times. Practice this several times a week and have the stick at a different location in the pen each day.