Teenagers - Where is the Line of Privacy?

I have reached the stage where my teen is about to become an ex-teen.
I have not been committed to a mental asylum - not officially anyway.
I have not been driven to the point of disownment and, as far as I can tell, my daughter still loves me as much as she did when she was twelve years old, before her teenage years threatened to make jibbering lunatics of us all.
I therefore consider that due to my finely tuned survival techniques, and although I have no professional qualifications on the subject of child psychology, I have some entitlement to express an opinion on the subject of teen privacy.
I consider that even though we have had our moments, I have been more than reasonably fortunate with regard to my daughter's naturally affectionate nature, which has seen us through bomb site bedrooms, Gothic fashions and cosmetics, and the complete destruction of all my attempts to instill a sense of responsibility with regard to how long one can leave a wet towel on the bathroom floor before mould develops.
I have found, through resisting every urge to trespass upon my daughter's privacy, that she has reached the age of twenty trusting me implicitly.
That trust is worth all the anxieties I suffered when she was deeply in love with someone far too old for her at the age of fifteen.
It would have been so easy to pick up her diary and read her private thoughts and assure myself she was being sensible, both emotionally and physically.
She was never one to leave her letters and diaries in a secure place because she trusted me never to touch them.
Instead I chose to talk to her and let her know that she could always speak to me about anything which frightened or bothered her, no matter how shocking or scary.
Not only that, she is confident that I will never force a confidence from her until she is ready to share a problem with me, even if I am bursting to know what is going on in her life.
She has repaid my trust in her, by trusting me.
We enjoy a deep and exceptional friendship and my main concern now is how I shall survive without her when she leaves home in a month or two.
It is supremely difficult to have faith in your children sometimes, but if you can maintain a level of respect for their privacy you will be rewarded handsomely in the end.