I'd probably start it, "I haven't been fully open with you about everywhere I've been going."
If you intend to change ... it'll be both tougher and easier. Easier to the extent that you're confident right now of your intent to change, but tougher in that ... you're likely to receive some adverse comments from them and some serious disappointment. The god of this world is famous for turning your confidence into being offended by some humbling comment from people you respect, challenging that pride that you're doing something right. Be prepared for that. I've always had to say to myself over & over, "Remember mike -- I'm the one who was wrong. Confessing doesn't make things right. It admits that I'm wrong."
Your parents likely love you a lot to feel such disappointment and stress that you predict will happen. It's tough for parents to recognize that ... you're human. You're going to make mistakes. And you're gettin' on up there in age, so there's not a lot they can do to help discipline you. You're going to be taking on that responsibility, yourself.
If it accumulates a lot of this stuff eventually takes you down spiritually and mentally. Plus, your parents end up not really knowing who you are -- your struggles, your aspirations. They end up knowing only your image as you present it. It sounds like they care enough to want to help, too. Would their help be considered interference or assistance by you?
Think about where you're headed, and how big the offense will be treated. My parents probably would've not been so surprised.