Do i tell information meant for ME, alone, to my girlfriend?

Ok, a person very close to me told me to be wary of because this person found it extremely difficult to stop once it became a habit. I told my girlfriend what this person told me, but she wants to know WHO this person is. I do not like keeping secrets from my girlfriend, but in this case, I feel that it would be bad to disclose this person's identity. Furthermore, it is not her business, and she KNOWS this person, so I don't want her opinion to change of that person. Also, no one wants OTHER people to know about their "personal habits" because it is very private. I trust my girlfriend and I know she will not tell her friends the identity of this person...but she said "It's ME, you know you can trust me!" Do I tell her? I've told her that I KNOW I can trust her and I know that she won't go blabbing about it, but this info still is none of her business and I am protecting this person's identity. What do I do?