delay(10); //Delay added to make thing stable
What do you think that this useless delay() is making more stable?
char c = bluetooth.read(); //Conduct a serial read
readString = c; //build the string- "forward", "reverse", "left" and "right"
You are NOT building up a string. You are OVERWRITING the previous string with ONE character.
You REALLY need to make your sender send some kind of end-of-message marker, and read and accumulate data until that end of message marker arrives. You are currently hoping that an entire message arrives all at once, which is not going to happen.
You need to spend some time with this thread:
http://forum.arduino.cc/index.php?topic=396450.0