What is her favorite thing, treats or toys? How is she on a leash? Can you lure her out with a treat several times, going further from the door each time? Poor thing, with such a painful condition, I hope it clears up soon. It is the front or the back door she balks at most? Does she usually enjoy car rides, could that be a lure to get outside, opening the car door and saying "Wanna go for a ride, Tessa?"

Mastiffs are stubborn, but also can be the sweetest big babies, I hope she feels better soon.