I have the same and my dentist gave me a mouth guard. You need to speak to them. If you don't have money for a proper mouth guard or medical aid, then you can buy one of those cheap ones that the sports players use at pharmacies, it isn't as good, but temporarily it is better than nothing.
Also to relieve you grinding you could put a warm cloth over your face for a while before you go to bed, that will help relax your muscles somewhat so that you don't grind as much.
I have a problem like that, only I bite down on my cheeks while I'm sleeping.. Usually if I have a really intense dream, I wake up with bloody/painful cheeks :(
You wont be able to actually control it without speaking to a doctor. But you can get a mouth guard so your teeth dont get really screwed up.
I have braces and bite the inside of my cheeks in my sleep also =[