WHO AM I?

I am your constant companion.  I am your greatest helper or 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 - but 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 place the world at your feet.  Be easy with me and I will destroy you.

Who am I? I am habit!