You can't stop her from making "her" mistakes...but you can help "limit" the possible negetive outcomes. When my teen-aged daughter became sexually active around 15-16 I took her to the doctors, she had her first pap test, and she was placed on the birth control patch...if you don't feel you can trust her to remember to use the patch correctly there are the shots that last 3 months...she may want to make her own "mistakes" but make it clear to her that they will be HER mistakes and that you will not pick up the pieces when things fall apart for her....if she wants to learn then let her face the full consequences of her actions. You can play a supportive roll as mom but you are HER mother... not the mother to any children SHE may have... practicing irresponsible behaviors.
She can bravely get in your face and talk about making mistakes and it's her life but until she is 18 YOU are legelly responsible for what goes on in this minors life and since I suspect YOU are footing the bills for her...room, board, clothes, shoes, money for extras, then you have alot of say in what goes on in her life. If she wants to be an adult tell her she needs to get a job and pay for the extras that she wants...ie cell phone, money for movies, dances, clothes, shoes, etc...as you can see being an adult isn't all it's cracked up to be...refuse to buy her anything extra beyond room and board until she agrees to live by your rules. And insist that she shows you the resect you have earned and deserve having raised and supported her until this point. Teach her now that every action has a consequence and make her live by them...and that she isn't the only one that will have to pay the price for her poor choices...then she will be one step ahead of her peers who think that they are "entitled" to things because they were born.
Let me ask you this...did you have all the extras growing up that your daughter has now? You survived and becme a loving adult because someone taught you about choices...now it's her turn to learn.