Why do I have sleep paralysis?

I have been having terrible sleep paralysis lately and its getting to be so bad that I don't even like going to sleep because i'm scared of it. For those of you who aren't familiar, its when you are asleep and technically you wake up...your eyes open and you can feel and sense the things around you but you are totally paralyzed...your eyes are the only thing you can move. Its happened several times where I can just feel a dark weight on top of me...and I can feel things in the room with me. I know a lot of people think this is just a dream and that i'm asleep but i'm not...nothing happens where my eyes suddenly open...they've been open all along. I just suddenly snap out of it sometimes. I would really like to know what I can do to stop it and what causes it. Its been happening since I was a little girl and I'm just tired of it.