You say 'safely reveal.' It might help us to know what you fear. Are you afraid of her leaving, her head exploding, hurting her? My father is fond of the saying, 'soft surgeons make stinking wounds.' I don't like it, but I know I've always preferred laser-clarity from folks when I've been dealt unpleasant news.
I can't say that I've been where you are. I have been a serial monogamist for a long time.
I can say that my boyfriends are fairly uncommunicative. They don't share much more than they have to. One of them is exceedingly (okay, that's my judgment) private. It's odd.
However, they are personally private. They're both pretty open to whatever I want/need to talk about; they just don't often have much to say for themselves. I have had to become more clear on when I need to talk, and I have had to learn that my happiness and well-being don't have much to do with whether or not they offer some part of themselves to the conversation. ach, I'm not being very clear. (maybe that's why they don't talk ~ they can't follow me!)
I guess what I'm trying to convey is that private doesn't have to mean uncomfortable. I can't imagine how painful it must be to have your partner not want to know your deepest self. I know other people in relationships like that. I don't know whether or not my partners *want* to know that, but what I do know is that they don't reject me when I offer it up.
It took us eight (nine?) years to get to our poly vee (and we weren't consciously aiming at it). I dated one first, the three of us were friends, broke up; dated the other; three of us continued friends, broke up with the second, and here we are in a vee. (just background so you know where I stand)
Wishing you best of luck.