Well, I got her in the snaffle and while she was fine with it in her mouth, as soon as I started asking her to move she started getting very anxious. Reverting back to her old spooky, balky, rearing ways, so I got off and started thinking about what might be causing her to be this way.
And then I realized something! I remembered back when the vet came out to do a check up on her, the vet had checked over her melanomas (gray horse tumors) and showed me that another common place for the to happen is the corners of the mouth and that Lacey had a few there.
So I decided to check them, and I discovered (I had never actually felt them before) that the corners of her mouth are literally rock hard from the tumors.
My guess is that when she has a snaffle in her mouth (she was actually a lot better when I rode her once in a curb which kinda makes sense) she can't feel the pressure on the side of her mouth before the bit pokes her in the rook of the mouth because "she's not responding," when in actuality, she can't feel that any response is necessary! Which then causes her to be nervous because she isn't sure when she's going to to poked "for no reason."
It makes sense that a solid curb would not get the same reaction since a curb doesn't rely so much on the corners of the mouth, like a snaffle does.
But anyway, that made me feel a whole lot better becuase I had been feeling like a failure of some sort since I can't keep my horse calm in a snaffle, the "gentlest" of bits for heavens sake!
I guess the moral of this story is that horses really do behave certain ways for a reason and that sometimes it's not something that can be trained out of them.
I thought I'd share because maybe there's another gray horse rider out there struggling with the same thing. :)