My suggestion is to first rule out any pain from the saddle. Ill-fitting or overpadded saddles can easily cause high heads and inverted frames, as well as tense, heavy, on-the-forehand halts.
If it isn't the saddle, reevaluate your transition aids. Be sure that you are riding forward into the halt, and asking with your seat and legs before your hands, like any other forward movement.
Practice your transitions, not just halts but every transition you can think of, between and within gaits. Practicing backing up can help as well, cuing off of your seat and legs and only using your hands to correct from moving forward or for straightening. Leave her head alone, and don't focus on where her head and nose are at. If you're riding her from back to front, the "frame" will come as her strength builds. Riding in a snaffle and fairly loose cavesson is fine; you're issue likely won't be fixed by martingales, flashes, or grackles whether it's the saddle or the aids.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown