I totally understand your problem. When I bought my horse, he was 3 years old, only been under saddle about 20 times and he was already being ridden in pelham! People who are into classical dressage always tend to use those kind of ''harsh bits''. When horses are ridden for many years with those bits they look like they have lost their sensitivity in their mouth, and when they start being ridden by good riders like you, who try to work them in a snaffle first, getting them working with it is a true nightmare for some of them! That's what happened with my horse when I first got him, but fortunately he was young and only being ridden for a month. But the fact your mare is already 21 doesn't mean that she won't get there. Here's something I would try to work in a case like this:
- ask her for a lot of transitions, transitions help you and your horse to improve a lot of things, such as getting her to feel your contact with the reins and the other aids, of course;
- work in a lot of circles, 20m, 15m, 10m, etc, and change rein with different figures such as numerous serpentines, diagonals, etc, it will help her feeling the bit and improve her outline;
- lunge her before riding with a good contact with the lunge line;
- when riding keep your contact flexible and elastic, I usually ride with a stronger outside hand and a more flexible inside hand.
I hope this helps and good luck :)