I also worried about this, but I still ended up posting my story on Fictionpress - only the first draft, though. So much will be changed second draft around that I'm not really worried. However, that's a big risk I'm taking. If you don't want to take that risk, you could always find some people in your area whom you know you can trust to help you through. Support, for a writer, is needed. It's a long journey to the end. Either that, or find one internet buddy you can trust who also writers to exchange chapters and the like through e-mail. Make sure they also feel strongly about copying and wouldn't do it themselves.
Still, there's a risk in everything. It's what makes life worth living.