Try a 3 piece bit, like a french link, they are much less harsh on the tongue and roof of the mouth, don't pinch. Also would suggest making sure the cavason is tight. I had a reg horse mare that did this once and they suggested putting the bit up a little for a while til the horse got used to not being able to put the tongue over. I am a person who rides "without" wrinkles, because I feel it is best to allow the horse a total release of pressure when they are correct, so this was hard for me, but it did work. We also tied a string to the center of the bit, I believe brought the string out both sides and tied it to the brow band or somewhere ( can't quite remember and that did not allow her to put her tongue over it. Was tied w/o pressure up on the bit. Didn't take long til we were just fine.
If you are using a side check or over check, keep in mind the side check is a bit easier on a beginning horse especially, and a horse that collects naturally.
Hope you come up w/ something that works.
Julie