Hate to break it to ya but I don’t think there is anything you can say that will make a difference. Even at 40 years old i did not want to tell my mother Riding bikes is a risky business. Telling her that sky diving is more dangerous will not likely help. At this point it seems like you will just have to wait, which is not necessarily a bid thing. I know that myself and some of the others on this forum have the opinion that if we had started riding when young then we probably would not be here today. Learning the rules of the road behind a steel cage does have its advantages. There will come a time when your mother will just have to live with your choices but at 17, that might not be just yet.
Have you considered getting into off road riding? That looks like a lot of fun and it would certainly build your riding skills. Come the day when you hit the streets (which is the dangerous part) you can concentrate on the road and not the bike.