I agree with @alixdubineau.
Changing the bit isn't the best solution. Do the exercises she recommended you to do and check with a vet as well.
To actually go into the bit part, I say it depends. You have to make sure the horse is comfortable and likes the bit, or else he or she will push against it(or show other signs of discomfort) and not make the situation any better.
I say start off with a simple bit like a loose ring snaffle or a D ring snaffle. Then if that doesn't work, slowly move up with the "harshness". Try a rubber pelham snaffle, gag, or waterford. Trial and error is the key to finding the perfect bit for a horse, you might get it after the first try or it might take weeks or months to find one.
If you have a trainer, I recommend you talk to him or her about it, they'll give their knowledge on bits and what they think you should try. Do your own research as well. See what your horse might like best and what each bit does.
I hope this helped!