Someone shouldn't have to announce to you that you can trust them. Like they say, trust is earned and actions speak louder than words. So she should do something that gives you a reason to trust you; for instance, does she keep your secrets or never let you down. If the answer is no, then you probably shouldn't be friends with that person. Trust is the foundation to any good relationship whether it's friends or bf-gf. If they tell your crush you have CF it's not the end of the world. My parents told my gf of 6 years that I had CF and I was so mad at them for doing it because I thought she might get rid of me. Well, I finally got the nerve to ask her several years later what she thought of me having CF and her response was positive. She said she was much more attracted to me when she found out because she had no idea how strong I was and understood why I did some stuff that I did. Also, it brought her closer to me because I no longer had to hide things from her and life was so much easier afterwards. The invisible wall gets broken down when your "secret" is out. It's not your fault and there is nothing you can do about it. Don't be ashamed of it, it's hard at first to know that people are weird about it, but they get used to it and as you get older they respect you more for being open and truthful.
Dave