I would say "talk to her"..... but the fact that she denied doing it means that she knows that it is a no-go area.
If you feel you can have a reasonable discussion about it, try telling her that you accept that she is looking out for you and wants to know if you get yourself into "trouble" (have sex, take drugs etc), but that you must feel you are able to have some secrets, for you own sanitys sake, and that if she has any respect for you she will let you make your own mistakes and learn from them, and she should trust you to be sensible and have good judgement.
However. Are you a rebellious, troublesome kid? Always chatting back and getting lippy? If you are, she may feel justified in poking around. If you improve, she should.
Or she may just be a nosy git. If that is it, cold shoulder to her. Most people will disagree here, but you need to make it explicit that this kind of thing is unacceptable and if that means making her life some form of hell until she gets it (I give her a couple of hours....), then do what you need to.
Diaries are private. End of.