Who am I...............................

I am your constant companion. I am your greatest helper or your heaviest burden. I will push you onward or drag you down to failure. I am completely at your command. Half the things you do, you might just as well turn over to me, and I will be able to do them quickly and correctly.  I am easily managed; you must merely be firm with me.  Show me exactly how you want something done, and after a few lessons I will do it automatically.

I am the servant of all great men.  And, alas, of all failures as well.  Those who are great, I have made great.  Those who are failures, I have made failures.  I am not a machine, though I work with all the precision of a machine.  Plus, the intelligence of a man. You may run me for profit, or run me for ruin; it makes no difference to me. Take me, train me, be firm with me and I will put the world at your feet.  Be easy with me, and I will destroy you.
Who am I?  I am a habit!

-The intertunnel is conflicted on who said this.  I found it here.