Your NO needs to be NO , not implied. Your benefit of the doubt is makibg you wishy washy which makes her keep trying to get whatever she can out of you & manipulate when she gets an implied NO because it really isn't a NO.

DBT's section on Interpersonal Effectiveness might be of great benefit to you.

A starting place to look at the topic....you can do more internet searches for more infirmation
Interpersonal Effectiveness