Try a Bosal or Hackamore on her. (Bitless bridle) I would make sure I put her in a corral or round pen before getting on to see if she is controlable. Some horses have had bad experiences from stupid people. Maybe a bit hurts her teeth/mouth. Just have patience with her!
Maybe her teeth need floated? And the bit is comming down on those sharp teeth and hurting her. May be start out w/ a less harsh bit too. I had to cover the bit in liquid molases once to get it in a pony's mouth. That is a last resort.
Who say' s not to look a gift horse in the mouth!!
I once worked a mare that did the same thing. I figured fooey on earing this horse and backing her into a corner to get a bit in her mouth, so I tried honey. The kind that squeezes out of the bear. Put some in my hand, shoved it in her mouth through the side, let her think on that a few minutes, then wiped the excess off on the bit, and slipped it in. Did the same thing a 2nd time, and by the third time, she was actively reaching to take the bit in her mouth. MUCH nicer than having to force it. Did the same thing a few times with my paint colt when I was breaking him. He now pickes up the bit out of my hand like he's taking a treat from it and I havent' put honey on it in 3 1/2 years. Whatever works and makes them willing to do your bidding instead of forcing it.
14 years old, eh? She's been there and done that, and may be sour on the bit, but it may be pain....always, always, when you have a problem like this, suspect pain first. I would get the vet to check out her teeth. At this age the tooth problems come into play moreso than a young horse and this could very well be the problem. Pain could also be caused by rough hands in the past, bridle too tight, more than one wrinkle where the bit sits, too harsh a bit or even just too old a bit or the wrong kind of metal and tasting bad, chin strap too tight. Maybe she just doesn't want to go riding but always check out the pain factor first. She may not want to go riding because the saddle hurts her and she just wants to avoid the whole experience. Horses are smart, you have to be a detective. If it is her teeth, and you get them filed properly, not only will it eliminate the pain of wearing a bit, but chewing as well. Her mouth could be causing her pain every minute of the day. So calling in the vet for an exam would be first, taking the bit out of her mouth in the meantime would also be good. A bosal is good, but can be severe and you'll probably have to go out and buy one....not cheap. A hackamore is just plain severe and how is simply clamping her mouth shut warrant that answer? If you do have to go out and buy something to replace the bit, then I'd suggest a sidepull. It's a glorified halter with a snugger fit for control. If she is a good riding mare with a good attitude, I'd just glorify my own halter and use that and avoid a trip to the tack shop. The sidepulls are not too expensive, though. Definitely check out the pain factor first, though. I have a 20 year old that has never had a bit in her mouth. She has the teeth of a 6 year old....the vet was amazed. She's needed help in the last couple of years, though. They do get long of tooth like the rest of us and I get her teeth checked every year with a filing needed about every other year.