Maybe try letting her out to run before you get on. Sometimes a horse needs to loosen up before they work. Lunging is ok, but letting her out to just run, I think, is better. They aren't confined to a circle and contently bending..they can just be a horse for 10-15 mins. Also, I don't know where you are from, but the ground may be too hard, and it hurts her feet.
There is more than just her back and teeth that could be hurting her.
Definitely get a trainer though, rearing can get really dangerous.