I'm going to go against the grain a little bit here, although I'm doing it with the caveat that this is about MY experience and ISN'T advice on how the OP should handle things.
I am a middle-aged woman now. I have a very responsible job, grown-up DC, a wonderful DH and a lovely life. In other words, my life has turned out fine.
This was not always the case. At 14 I went right off the rails. I got kicked out of school, fell in with the wrong crowd and did what I wanted. My poor DPs, who had no such issues with my siblings, had no idea what to do with me. I too had a diary. Some of it was true. Some of it was an embellishment - a sort of 'this is what I think would have happened if I was an exciting and edgy person' version of events. My mother found and read this diary and shared it with my father before they both confronted me.
Yes, I railed against them for invading my privacy. However, deep down I was glad (although it took time to realise this). I had lost control of my life and my parents stepping in and reasserting the boundaries helped press the pause button long enough for me to take stock and reinvent myself.
Throughout this whole process my parents weren't angry with me, just disappointed and concerned but very much still loving. They made no apologies for reading my diary saying that they hadn't gone looking for it but on finding it were obviously going to read it. I was a child, they were my parents, their job was to protect me and as such children don't have the same rights to privacy as adults (although I was 14 and your DD is 16 so that's a little different).
It didn't ruin my relationship with my parents. It didn't ruin my trust in them. In fact it reinforced it as I knew they still loved me in spite of it all. It did teach me to hide things better though...
There's been some good advice from both ends of the spectrum on this thread. So much depends on context.
Most 16 year olds do not smoke weed regularly. Many may have a go in group if a joint is being handed round but they don't keep their own supply and this is still a minority, albeit a sizeable one. Regular cannabis use at 16 is something to be concerned about, but even allowing for the significant increase in strength since 20 years ago, the odd toke at a party is unlikely to turn a 16-year-old into a drug-addled psychopath so perspective here is important.
Similarly, I undoubtedly went along with sexual activity I didn't really want when I was a teen. I was not raped. I would not describe myself as sexually assaulted either. I probably gave the impression I was enjoying it because I was desperate to portray myself as a cool sophisticated woman of the world. You can blame the patriarchy for that (and I do 
) but not the equally inexperienced teen boys who had about as much of a clue as I did. None of them hurt me or coerced me. I'm not scarred and have a very healthy sex life and attitude towards sex. Sex at 16 is common. However, it can be a wildly different experience depending on circumstances. A BJ on a park bench would concern me. It's not private and suggests quick gratification for the recipient. It's hard to imagine what an inexperienced 16 year old girl gets from that. She could like the thrill, she could have been manipulated into a situation she didn't know how to get out of, she could have been forced.
The self harm is what I'd want to know more about. Do you see your DD in underwear? Have you noticed any marks? Again, it's surprisingly common and doesn't mean the child is doomed to a lifetime of mental health issues. A lot of teens grow out of it. They use it as a form of emotional regulation until they find another, better way. Some of them do it because they think it's a bit dark and edgy and get bored eventually. Sadly, for others it can become a problem and is a real cry for help. None of us can know what's going on here.
You know your daughter Bert than any of us OP. You're going to have to decide how to handle it. Whatever you do, it will be the reassurance that you love her and support her no matter what that's remembered and will make the difference in my view.
Hope it all works out. 💐