Well, stupid me tried to fix it anyway.
My TWH was started with a walking horse bit 6 months ago because that's what friends told me I "had" to use on a TWH. She's very responsive to my cues and neck reins wonderfully. But after reading a lot of opinions about bits and being told that you should never start with a curbed bit or neck reining I decided to buy a french link snaffle and give it a try - to do things the "right way".
I recently started using a new girth on her. The neoprene one I used had been tightened too tightly once (by someone else) and she got rubbed by it. So I decided to start using a "gentler" non-neoprene girth as well.
I'm an idiot.
As soon as we began she started going way faster than I wanted on that road. She was bouncy and rough, not her usual smooth gait. I couldn't get her to slow down or stop for anything. Then the saddle slipped to the left - I tried to hold on and stop her, but realized I was better off to bail - I was pretty close to the road since the saddle was sideways. I didn't even get the slightest scratch. Apparently, I'm pretty good at falling.
She stopped right there. I got up and walked her back to the house. I put her usual bit and girth on her and we rode in some nearby fields for about an hour. She was wonderful, responsive, and smooth.
Her curb bit (with a sweet iron french link) is definitely gentle enough if it is what she responds so well to. I know that the harshness of the bit depends mostly on the hands of the rider. I barely have to move to cue her in it, so we won't be changing again anytime soon.
I guess she likes what she likes. Who am I to argue?