When you say she behaves 'relatively well' around you and your grandpa, is it nervous behaviors or is it aggressive behaviors?
If she's nervous, try to expose her to everything possible, make sure when you do though that you stay calm on the rope. The calmer you are when introducing new things, the calmer she will be. Playing a radio in the barn does wonders. When I'm breaking calves, I will have it loud and will change stations every so often so she gets more exposure to different voices, noises, etc. Something else that might help is having someone come out and help you with her. Just someone new for her to get used to
If she's aggressive, I.E. if she takes off, pushes you around, etc, she may be better off getting turned out to grass and let be a cow.
A donkey/mule won't do much as far as getting her used to being around people.