That could be a place to start, just by letting her know that you'd be more open to sharing if she makes the effort of asking.

I did. I told her she could have just asked me. I told her I might have shown her the document if she'd ask first. She just shrugged her shoulders (literally) and later stole it back from my room.

The thing is, what else can I do? Tell her to stop and explain to her why she needs to and how it hurts me? I've done that so many times. Actually open up to her? When I did in the past more than once, she told her relatives and my friends mum about personal information. I don't trust her at all. She's told me things about my sister and said "don't tell your sister this because she told me not to tell you" etc.