I used to have an appy who put his tongue over the bit, and because of it he would get cuts in his mouth/tongue and become very difficult to steer etc. He was abused previously, so I think it may have been a stress thing. Here in Australia you can buy a rubber peice that fits onto your bit, it is about the size and shaope of a human tongue and it liesflat up the horses tongue, much like the tongue control bit, but a lot cheaper! I used this until he calmed and stopped attemping. I then moved onto a kimberwick with a medium port, which still gave him that little bit more trouble getting his tongue over, then finally I moved on to a rubber mullen mouth snaffle with a cavesson noseband. By the end he was fine in the snaffle and noseband.
Good idea to stop riding until you find a solution/training aid, as it does bad things to their mouth and tongue. With spot (original name for an appy!) it was not a training thing, or a disobedience thing, I think it was simply a ngeative reaction to having a bit in his mouth. Progressing through the bits, lessening the tongue control as he got used to it, was really the best way to go.