You are keeping a family secret from your daughter and this is inappropriate. You read something of hers without permission. I would share this privately with your family counselor to discuss the most appropriate way to confess to your daughter. Your daughter is likely to become enraged by this invasion of her privacy and it may temporarily derail the therapeutic work you are doing. In the long run, however, she will respect you more for admitting this mistake and asking her forgiveness. She might continue to deny that these things ever happened. When we are caught in a situation like this, we tend to get angry, strike out, and deny the obvious.
I am concerned that your daughter may be approaching sex as a casual recreational "activity," void of emotional commitment or mutual respect. Her drug intake is illegal; what's even more troublesome is whether your daughter is using drugs to cope with her day-to-day life.
Any discussions about sex and drug-taking, with or without the family counselor, need to take place without shaming your daughter. She'll be on the defensive and needs empathy, not condemnation. I'd also recommend that you read, Venus In Blue Jeans: Why Mothers and Daughters Need to Talk About Sex, by Bartle and Lieberman.