Just because she's old doesn't mean she was taught to give. ;) A horse should do it in any bit; if they don't like the bit then usually the response is faster, NOT resistance. This is because the horse wants to get rid of the stimulus as soon as possible.
From what you are describing of her actions, it sounds like she does not know how to give (She starts freaking out and backing up... not signs of a horse 'broke' to the bit!). It's not something terribly hard to teach.
However, bits with three joints (Not Dr. Bristols, but french links) tend to be comfortable IF the horse dislikes pallat pressure, and likes lip, tongue and gum pressure. The waterford, again, is another bit that has a lot of bite (because it has several joints), but used softly can be extremely comfortable in a horse who likes tongue, gum, and lip pressure. It starts to get confusing when you look at your options. ;)
If I were you, I would look for a french link, and then start training her to give to the bit. I don't think the bit will solve any problems, but the french links tend to be kinder, which will help her learn.
Dressage in Jeans
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