Originally Posted by stevenson
the best and worse bit is determined by the riders hands. A harsher bit is not your answer. Work on making your contact light, get collection with your seat and legs. If your trainer cannot help with this, get a new trainer, not a new bit.
This is all well and good, and I agree that a rider should work on making their horse more adjustable and attentive, but sometimes a responsive horse in the ring can still get excitable and might need a different bit. Note that I said different, that doesn't always mean harsh or stronger, just something that works differently in the horse's mouth.
OP, I think you should perhaps try a thinner snaffle first before phasing out the snaffle for cross country. I suggest a Myler level 1 loose ring. They're fairly thin and if she goes well in that then great! A Myler combination is a bit strong to automatically switch to. From your description, it sounds like she's just running through your hands and ignoring you. Do what stevenson suggested and really work on her attentiveness and adjustability outside of the arena. I don't think she needs an elevator bit, just something with a little more oomph to get her attention.
If I were you I'd try a fat, slow-twist full cheek, keeping in mind that it's definitely more bit than you're probably used to and you need to be very careful with your hands. Does she pull down at all? Or is she just ignoring your direction and not turning when you want her to?