If you keep going from bit to bit you'll never solve the problem. If she is heavy in the hand and she pulls then you are giving her something to pull against. She has to learn to carry herself, not to rely on your hands, and to respond when you use the reins. And you need to teach her this by not pulling back.

Do lots of basic schoolwork and really concentrate on having her on a very light contact. If she starts to lean, give and retake the reins so she has to hold her own head up. Do polework and lots of transitions to get her off the forehand. Flex her neck to each side, teach her to rein back (lift your hands up as you ask or she'll just lean on the bit). Make sure your hands are soft and light and giving, but with a steady contact. If you accidentally jag her mouth or have unsteady hands, especially in a harsh bit then she may grab the bit to avoid sudden tugs.

Change your bit to something softer, have some lessons and concantrate on her carrying herself, not leaning on you.

i don't believe in stronger bits....re-school her and work with her ....getting her to expect the bit with schooling ....i don't like to use gagits but draw reins to get her working on the bit helps .....if your horse has built up mussel under her neck she will not work with the bit ...i had the same trouble with my lad ...i schooled him in draw reins and free schooled him in them for about 4 months ....to build his neck up on top and to help take the contact  ....he can be strong still but using my voice and body he now works lovely .....if out on a hack i never give him contact and like him to have his free time ...playtime so to speak .....i ride western out and english to school and at shows ....he now knows the second that english saddle goes on he has to work ...if you get her working with the bit she will respone to it .....stronger bits will only restick her in her work and make her more stronger in the mouth.....get working together and i promise you , you will see a big difference .....he need be email me any questions ...good luck helen xx