I'm hesitant to offer my two cents, I don't know either you or this N, and I haven't been in a domestic discipline arrangement like you had with her. My relevant experience here is a former significant other with a certain fixation, and that might be similar enough.
As you described it, N was using DD to manage her urges and modify her behaviour - things which are typically part of a therapist's bailiwick (And you mentioned she had/has been seeing a therapist as well, which solidifies my theory a bit). A younger N might have seen someone like you as a therapist-with-benefits at the time, and then was left feeling used when you wanted to change your dynamic for your own gratification. Understand, I think you did your due diligence as a kinkster, but she wasn't approaching your relationship as a kink.
How to avoid this situation? I can't say for sure, but it would help to establish what both parties are getting out of the DD arrangement.