A question about making friends....?

Because that is such a personal subject and if you think about it, it's really no one's business but your own, I would not tell her. Should the subject ever come up, you can THEN tell her the parts that she needs to know. But you are right about it possibly hindering your relationship. That kind of information sometimes scares people into acting out against you, or forming opinions that they aren't fully educated on. Tell someone you had a stay in the mental hospital and they picture straight jackets and electroshock therapy, ya know? You don't know her well enough yet, and it's better not to risk putting it all out there on the chance that she will not last as a friend. Give your friendship time to grow, and if she proves to be trustworthy, maybe one day you can tell her about your "vacation".