My mom won't let me buy a menstrual cup??

So I've always have had pretty heavy periods right...ok so I use a tampon and a pad and still manage bleed through my pants ALMOST EVERY TIME. I figured getting a menstrual cup would be best. I tried telling her that I would like to get one(with my own money)....but she just told me to just deal with it and that menstrual cups are stupid and that I should just change my pad and tampon every hour. Idk why she's so weird about this stuff. Mind you she wouldnt even let me buy tampons until I was 15 bc she was convinced I would get TSS. What should I do? Should I go out and secretly get one myself? How do I convince my mom that a menstural cup is better?