I know the man who bred, raised and trained this mare. He has never put a bit in any of his horses mouths. He trains all his horses in a mechanical hackamore. When I first got this mare, I tried to ride her in the hack that she was used to and she nearly went up and over with me, hence the reason I tried all sorts of bits, and since she wouldn't accept those, the reason for the softer hack that she likes.
One of the enduring "fantasies" of equine training is that "mechanical hackamores" are less "severe" and more "humane" than a bit. They are not, on either count.
If this mare was never trained in bit then my advice still stands. Another training myth is that we humans must only use things the horse "likes" or "will tolerate." In truth we humans decide what will, or will not, be used. It's up to us to be smarter than the horse and figure out a way around the horse's resistance. Again, with an older horse this will be more difficult.
One thing you might want to do is have your vet check the horse's mouth conformation and dentition. One valid reason for using bitless devices is mouth pathology or conformation. It's possible that some abnormality exists that will make bit usage problematical. If you've already done that you can check it off the list. If not, I'd get it done.
If you want to go away from the mechanical hack but feel that your skill or experience level is insufficient to take on the task then you'll have to "hire it done." You can engage a trainer and have them do it or find an instructor that will work with you and teach you how to teach the horse. That last route is slower and more work but you will add to your "skill tool box" and might well benefit in the future from what you learn now.
This task will require the investment of some time and money. Only you can decide if the payback will justify the investment.
Good luck in making your decision.