Mom is supposed to be overly protective. That's in the job description. Some tips to try to reduce her concerns:

- Plan plan plan. Have your route picked, with overnight and food stops selected.
- Have emergency plans for unrepairable breakdowns, injury, illness.
- Have a cell phone and a way to keep it charged
- Think about every worry your mom will come up with, and have some plan to address.
- If you could have "safe houses" along the way - friends or family, that could alleviate fears

Have all this done before bringing it up again. You may be almost 18, but in her eyes you're still 8. You've got to impress her with your planning/preparation.

Or you could just go, and deal with the consequences later. But really, if this is your first long solo tour, overplanning is not a bad idea anyway.