So you’ve been monogamous with your GF for half a year and she is insecure about relationships you’ve had prior to that? Ask the question exactly like that. Emphasis that you have been faithful to her for the entire period of your monogamous relationship and that you can’t help that you spoke with/went on dates with/ slept with other people before you made a commitment to her. You can’t tell the future, how were you supposed to predict that she was going to be someone you wanted to commit to and Girl B wasn’t? How were you supposed to think that she might be “the one” before you even met her?

Does she honestly expect, as an adult, that her “Mr. Right” has never dated anyone before and is a virgin? It sounds like she might have some sole searching and some growing up to do. I wouldn’t worry to much OP, just know that if this can’t be resolved it will be the end. Of course that will only free you up to find a relationship that works.

I’m not trying to forecast anything. Just explaining things as I see them.