It's easy to be so afraid of making a mistake that you never get around to stepping out on faith. You can spend all your time wondering, "Is this faith? Or is it presumption? What if I exercise my faith for something and find out later I've missed God's will?"
Don't worry. God can handle any mistake you can make. I know because I've made plenty of them. When I did, I'd just go to the Lord and He'd tell me, "Stay on the Word, son. Together we'll overcome this thing." And we always did.