The 09 and 10 bikes came out with soft springs that normal suited light riders like yourself. There are two issues i get from your posting.
First..... If you have the preload on the spring tight up, it is likely you have the wrong spring or perhaps the spring is shot. You should set sag to 1/3 travel when you are stood on the bike with riding gear, after this there should be a little static sag (10mm ish) (with out any bike load)
Second.... The shock screw adjusts only the rebound speed. If you get no adjustment when you turn the screw - your shock needs a rebuild. And this is likely to be the reason your bike is blowing through all its travel very easily. But it will be doubly bad if the spring is shot. It should take a LOT of force to compress a shock.
You can rebuild what you have but it is a huge challenge to find someone to do it, and get new spring, or if you can afford one buy a TRP shock they are brilliant.
Let me know what happens, Ralphy