I'd start small--don't feel like you have to come clean about it all at once. Maybe start by saying what you wrote here: "I feel like I’ve held back so much and I want to tell you stories and be honest but I never think it’s the right time or I don’t know how to bring it up." Print it out and hold it in front of you to read to her, if that helps. And write a list of things you want to talk about--I've found referring to that helps keep me on track (T will ask me if I have my index card of topics with me).