Why does it matter if your mother says "no"? If you say "yes" what will she do? Even if you're on her health insurance policy, she can't keep you from seeking medical treatment after age 18. At least that's my understanding.
Why don't you call a treatment facility and speak with someone? They will be able to help you assess your needs better than we can or your mom can. Or maybe work with a substance abuse counselor to get some feedback.
Good luck!