How can I manage my own screen time without signing out of Family Sharing on iPhone?

I’ve just turned into age 18 two days ago, and I know that screen time management by myself would be possible in that age. But why my parents still can manage my screen time and I cannot even sign out my family sharing. I just want to manage my screen time by myself and don’t want to sign out my family because of the iCloud. What should I do?

[Re-Titled by Moderator]