TL;DR: I've been there and I'm so sorry you're going through this. Make it a priority to take care of yourself.
Long version (sorry I went full advice-auntie mode, but I'm posting anyway for all young period-chans out there):
I started having cramps at age 10. Every month I'd throw up from pain, was doubled over, couldn't see straight, you know the drill. I also had a ton of bleeding. In HS I saw a gyno, and they told me "this might go away after puberty or it might not; if not, get checked for endometriosis." So I got diagnosed w/ endo in my early 20s. BC helped the bleeding but not the cramps. I also had PMDD and everything I tried either didn't work, stopped working, or made other issues worse. When I came to the end of the road for other viable treatments I decided to have a complete hysterectomy. One of the best things I ever did.
So here's some advice. My apologies if you already know all this stuff/have tried it, or if it doesn't apply to you.
Plan to get on your own health insurance as soon as you can, find a Planned Parenthood, whatever you gotta do so your mom isn't part of your medical decisions anymore. Until then, get her to take you to a gynecologist if she hasn't already. Maybe an MD can convince her to let you try BC.
The cramps and bleeding may lighten up by themselves once you hit your early 20s. Or not. There's no way to know.
If you're missing school from cramps every month, try to get a diagnosis in writing, excuse notes, anything you can get from an MD. Keep copies of everything. Even if you're only missing a day a month, that can snowball on your attendance record very quickly esp. if you have other issues that also make you miss days. If you can prove absence was due to a medical condition, it can help you prevent the school from fucking you over. One semester I missed so many days, my doctor had to write a letter to the fucking school board because they wouldn't give me credit for my classes even though I had straight A's. Whatever it takes. If you are still having disabling cramps when you get out in the workforce, that paper trail could save your job. Don't let men and judgy, cramp-less women screw you out of anything just because "oh it's just cramps, it can't possibly be that bad." Fuck them, protect your rights.
If you take NSAIDs like ibuprofen and naproxen, never take more than you need. My stomach was trashed by the time I was 30 because I'd been taking shit-tons of Advil every month, and now in my 40s there's a ton of stuff I can't eat and meds I can't take because my stomach lining sucks. If you are prescribed something for cramps, be careful with that too. Honestly pot can be really great for cramps, just be mindful of the laws where you are if you decide to try it.
Learn everything you can about pain management. Every trivial-sounding thing, from getting enough sleep to self-hypnosis, could help you deal with the pain. Try it all, use what works, ignore what doesn't.
And get tons of iron. There are supplements but they'll constipate you like a plugged cement mixer, so try to get iron through your diet if you can.
I wish I had something more immediately useful to tell you, but I hope this at least helps a bit.