Please contact this rescue group, they can find a suitable home for your pup:
United Yorkie Rescue - A 501(c)(3) Non-Profit Yorkshire Terrier Rescue Organization
A yorkie will not trust you if you hit/yell or get angry with it, they are smart dogs and know better. Maybe your fiance's ex bf hit the poor girl as well, and that is why she is not trusting of you, she may have fear in her because of past experiences with men. If she is causing this much conflict between you two she should be allowed to find herself a new home with people who are compassionate of this breed, and will work with her to find the solution to whatever is causing these problems.
Graduate school is a lot of work, as is working towards your bachelors, you fiance needs to see that she may not have enough time for this little girl and every dog deserves a family/home where she is the center of attention or at least gets enough exercise and attention so they don't have to act out to get the attention they crave.
Please, for the dogs sake, contact a rescue so she can be placed in a home where the people have the time to train her properly, which at her age will take a while.