Have a good talk with your trainer to change up the routine, not the bit just yet. Train her up from the hind legs and she could get lighter in the hand in front. Good luck!
You shouldn't need a stronger bit, honestly. The best way to fix a horse that's heavy on a bit is to go back to groundwork. Any reputable trainer will tell you the same as I had the same question a few years back and know from experience. Best of luck! :)
A friend of mine had good success with a Waterford for her extremely strong mare.
You may want to try a bit with poll, cheek or chin action as she isn't respecting the bit. Also try half halting every other stop if you don't already. As a general rule with strong horses, I like to remember "if a half half doesn't work, do a full halt", that way they should start better respecting your hand and seat.
I sometimes have to use a junior cowhorse bit for one of mine...it's broken like a snaffle but it has a little bit of leverage