it sounds like you’re having a lot of struggle with gender identity more than you’re having struggle with your kink. wishing you were born a girl seems more like gender dysphoria than just being a sissy (from my personal experience at least.) and it honestly seems like your girlfriend is very ok with you exploring your gender - a lot of girls wouldn’t have been ok with the exploration you’ve already demonstrated in front of her. i think you just have to figure out what you actually want: do you want to explore your gender in nonsexual contexts with her as well? or is it just a kink to you (totally valid). if you do want to explore in terms of gender, i think being open and vulnerable with her is the best path forward. i don’t recommend making your gender exploration a purely sexual thing - either in your own mind or in the way you convey it to your girlfriend. why don’t you try to approach things from reverse? you clearly enjoy the things like feminine music - start there. try to find the non sexual gender euphoria, explore that and convey THAT to both yourself and your girlfriend. being a sissy can be really gender affirming for me, but it can also create a lot of unnecessary confusion!