Have you talked to your mother/female guardian about it? Some women have a much heavier menstruation than others. I've had some friends end up getting The Pill prescribed to them to help regulate their period. Ask your mom about seeing the Woman's doctor to talk about regulating your menstruation.
Women who do a lot of athletics do tend to develop more yeast than others, which, from what I've read, makes a heavier discharge. Because of my mother's heritage, she's passed on to me the practice of how Filipinas wash after every bathroom use. I forgot the actual term. If you want, try to do that. Sometimes I use a large plastic cup I get from fast food places, wash it out real good, then fill it up with very warm water, and wash myself thoroughly. You can't be afraid of your own body. It's yours, after all, lol. You won't harm yourself, and it's perfectly modest. You're just cleaning what God gave you, lol. Doing this often keeps you from getting a bad yeast infection.
Make sure you have a nice, healthy diet too. If you want some tips on that one, send me a message and I'll gather up some good resources. I do know that Body for Life for Women is really good. Even if you're a teenager, I think it's just right.
I will say that my period did get a bit lighter as I've gotten older. I'm nearing my mid-twenties, and it went from being horrible to not as horrible. Usually being athletic helps it become less heavy. Do you also have a lot of tissue during your menstruation? My heavy flow days tend to have a lot of tissue.
Anyway, I hope anything I've said has helped. I do strongly encourage you seeing a Gynecologist. And never forget to talk to your mom. Or someone like a mom to you. It's never healthy to hold anything in, because stress can cause any type of health problem, lol.
Hon, talk to your mom. She may have had the same problem when she was younger. She may have some solutions for you. If she doesn't, she'll know where to turn to get the answers, like a gynocologist. That is a doctor who specializes in female stuff. That doctor should be able to help you out too.