Well mine would toss and bob his head and my trainer said getting him to drop his head will get him to give and be softer on his mouth. It works he doesnt toss or bob anymore I do it every time before I ride. Just make him give and once he does praise him and do it 3 to 4 times.
Get her teeth checked. I have a gelding who was a terrible head tosser, and he still had his wolf teeth at 12 years of age. The vet floated him, pulled the wolf teeth, and cleaned up an abscess and no more head tossing.
The head tossing sounds like pain somewhere. I would have her teeth checked and then check all of your take to make sure it doesn't have pokes and that it still fits correctly. Sometimes when horses loose or gain weight it can cause the saddle to no longer fit and cause pain. There could also be an underlying health issue somewhere as well that hasn't made your horse sick, but uncomfortable such as tummy troubles or even ulcers.... my horse was a BAD head tosser and it turned out he had an ulcer problem. The head tossing completely went away after treatment.... so just be aware that there could be something hurting your horse that you can't physically see.
When you are positive it isn't pain or tack fit, I would look at your bit and make sure that it's not hurting your horse. Maybe you might need a "friendlier" bit.