Hey,
I can understand why you would like to be able to fully explain your position. That would be so cool! But for me, I've had to realize that I'm not perfect and sometimes the only way I can set a firm boundary is to simply say, "I don't want anything to do with you. Leave me alone." Of course she's gonna try and talk you into staying! All JW's are trained to do so.
Just remember that you are under NO OBLIGATION to explain anything to her. It's not like other relationships with, say, a spouse, where the person DESERVES an explanation. I'm sure she told you that "any time you want to stop the study, you can." If she starts trying to bully you or emotionally blackmail you (this one more likely), tell her to leave immediately. And if she implies, "well, I guess you don't love Jehovah or appreciate what he's given you," tell her to f**k off!
Oops, sorry. Don't mean to be bossy. The sad truth is that this woman probably doesn't really care anything about YOU. You are just a notch in her belt, plus easy time to count as service hours.
Be Brave and Firm!
Dinky