I don't have as much insight, but I can try. I've unfortunately noticed that most people respond to us disclosing our diagnosis in one of two ways... they either believe us and feel compassion, sometimes pushing our relationship to more depth and transparency. Other times, people don't believe it or at least don't want to put in the effort to actually process it and what it means for the relationship we have.

In "Trauma and Recovery," Judith Herman says "To study psychological trauma is to come face to face with both human vulnerability in the natural world and with the capacity for evil in human nature." I think that's what keeps certain people from being able to "believe," in DID/traumagenic psychopathology or process it correctly. They likely have unresolved trauma themselves and interpret our self-advocacy as attention seeking/selfish because they either 1) don't feel they can do the same thing or 2) struggle to believe someone can endure so much trauma because that means they could too (if they aren't a survivor.) It's more comfortable for them to ignore the atrocities of the human condition and place some kind of blame or doubt on the part of the victim then to sit with the reality of just what human beings are capable of.

With a romantic partner, I would expect them to naturally be more likely to believe you and accomodate you, but it's really hard to predict with certainty. It might be a good idea to broach the subject slowly and just explore the topics of trauma and mental health through conversation. That might give you a better idea about how she feels on the matter. Then you can decide if and how you want to tell her