Permanent Fix
If you want to fix it permanently, you’re going to have to put and keep your leg on her even though she’s curling up on you. Heavy/curling/leaning at a gallop is essentially the same thing as being on the forehand on the flat.
I fought this battle with a mare for years and have a big bag of bits you’re more than welcome to shop from, seeing as how we’re both in Alabama!, but I didn’t really fix the problem until I figured out how to move her shoulders while keeping my leg on. Just ask her to move over a step as she’s curling, and see if that gives you 1 or 2 unlocked “up” strides. If so, keep reinforcing and refining that aid.
The reality is that you’re probably not going to get them to actually move over as they’re galloping, but the effort to do so does kink the otherwise kinetic chain of energy (the locked and loaded feeling), giving you a trainable moment.
As far as straight-up bit recommendations go … I have 2 Waterfords … a loose ring and a Baucher. I didn’t find the loose ring to be too effective, but the Baucher seemed to do the trick.