Moje uwagi:
I used to have this doctor that I'd never go back to her. - myślę, że trzeba usunąć "her" ....
TAK. To zdanie powinno sie konczyc na ..go back to......
I used to have this doctor that I'd never go back to
her. - tutaj tez trzeba usunac 'her.
I used to have the doctor that I would never go back to. (To zdanie jest niegram), tutaj musi byc 'a')
I used to have a doctor that I would never go back to, - zdanie ok