She's a friend's mare and had been race-ridden many years, speed demon. She is ridden with a smooth snaffle, shall I continue using that or would bitless be better? She has been ridden in both
The answer is to stop riding with your hands and re-train her. Use lots of lateral exercises, using mostly your legs and only use the reins if you HAVE to. If she takes off pull her head around with one rein and have her disengage her hindquarters (kick out her hip) to get her to stop. Use a smooth snaffle or a dogbone snaffle when riding to help prevent excessive pressure in her mouth. She needs to learn to be light and responsive to the bit pressure, but the only way to do that is for the rider to have impeccable feel and know when to give the release of pressure.
There are two things that could be happening. One, she could have something wrong with her (ulcers or a small injury) or two, she could just be not listening. You could add some training tools like a draw reign or a German martingale or change your track. For example work in a small area so she doesn't have the chance to run away.
Lots of ground work with the bit. Disengaging and all that jazz👍🏻
You have to soften her vertically and laterally and the rest will fall into place
With horses that don't want to stop or slow down slots of half halt and alternating pulses. Pulse the rein with the left hand then the right, and start light and work your way up to tighter pulls if she won't listen. Also can use your seat to slow and small circles work really well for a horse that won't slow down. Usually it's not a problem with a bit but a pony just not wanting to listen so small circles anytime they won't slow after you ask will make them learn that if they don't listen to your rein aids they have to work harder.
Maybe try a hackamore? My horseback was dead to the bit so I switched to a hackamore and he became much happier...if that doesn't work I would suggest going to a simple snaffle and restarting at basics?