I have a question, why do you pony the bay horse every time you go riding? Has your horse always been buddy sour? I don't see how you can work your horse and get over the herd sour problem when leading the other horse. Leave the bay with the paint and see how she acts, if she doesnt leave I agree with working her hard near the paddock I.e. Yielding, circles, backing etc and resting away. Do you ever ride the bay?
Dickson and Tracy ... Our horses
Dixie (5yr. old QH) Cheyenne (9yr. old Paint) Uno ( 11yr old Arabian)