This is so effing effed.

Did you get anything in writing regarding the fact that she was going to hold back on billing until you were more financially secure?

They took so much advantage of you. It's like, if you were anyone else - say a man who has rage issues - their lives would be endanger over this. They continued the cycle of abuse. Are they part of a practice or anything? I would be in touch with the practice or the ethics board and file some formal complaints.